The d20 System is Wizards of the Coasts' own attempt at creating a Generic Roleplaying Game. Like many others that have come before it, such as GURPS, Palladium, Hero System/Champions, and Fuzion, it has its own strengths and weaknesses. However, since the d20 System is based on Dungeons and Dragons (specifically, Dungeons and Dragons Third Edition) it has a new take on the system. I call this node the "strengths and weaknesses" mainly because I know that the system has both, but I mainly expound upon the strengths of the system because I think many people underestimate them. The weaknesses of the system are rather esoteric in some places, but I will do my best to explain them.
First and foremost, the biggest strength of the d20 System is familiarity. When you mention roleplaying games, you frequently have to add the statement "You know, like Dungeons and Dragons" to your explanation. People recognize that name. Unfortunately, they mostly recognize it due to bad press, but they do recognize it. (That notoriety was not entirely a bad thing for the industry.) No matter how good your system is, if no one has heard about it, no one will play it. The strength of that Brand, as Wizards of the Coast calls it, will improve your game. Since WotC is filled with very good businessmen, they take this very seriously. They have a Brand Manager in charge of the d20 system. So, the logo, the SRD (a document which details what you're allowed to use, and not allowed to use, in your game), and every little detail of the system is carefully planned. Once you get your game "up to code", you can place the small "d20" logo on the front of your game. Gamers like me now recognize that symbol. Steve Jackson Games is now trying something similar to this with their "Powered by GURPS" campaign, but I don't think they have quite as deep pockets as WotC. And even given the same amount of cash, WotC is filled with very astute businessmen. (And since they are backed by Hasbro, they do have quite a bit of cash.)
That brings me to the second benefit: stability. GURPS is owned by Steve Jackson games, a much smaller company. GURPS has run into financial difficulties in the past, had to lay off many of their employees, and in the end was only saved because Steve Jackson put his own money back into the company in order to save it. Steve Jackson loves the industry, and his game, so he saved his company. Any other company manager would have liquidated the company and cut their losses. If Steve Jackson had something happen to him, or the industry took a big hit, or any of a number of things, the GURPS books might stop being produced. And Steve Jackson Games is one of the bigger companies out there. Hero Games took years before they released their Fifth Edition rules, so many years that many gamers gave up on it ever being released. Iron Crown Enterprises (makers of Rolemaster and Middle Earth Roleplaying Game) went completely out of business and sold their assets to another company that was founded just to prevent the rules from dying. Despite all the industry movements, Wizards of the Coast will prosper. And your players will be able to walk into any gaming store and pick up a copy of the Players Handbook and your game, and hopefully play.
A tremendous strength is familiarity with the system. I can't even begin to count the number of systems which teach me new tricks to play with dice. Every system seems to want to incorporate a new dice trick. Roll X dice, add them together, count numbers above a target number. If Y shows up on a die, reroll and add, roll another die. Reroll 1s, don't count 6s. Roll d10s, d6s, a d20. Heck, Fudge even invented a new form of die just for itself. I'm not here to play Dragon Dice, I'm here to tell a story. Or I'm here to make some numbers go up. Almost all of us who play RPGs started out with Dungeons and Dragons, though. So, WotC distilled the essence of the D&D system into a simple mechanic: roll a d20. High is good, low is bad. Always. You do roll dice other than a d20, but if you ever have to guess, a d20 will be the right choice. John Wick once said that all RPGs have a grand total of two mechanics: swinging a sword and picking a lock. And for both of these two choices, the d20 system is simple and familiar.
This is also the strongest drawback to the system. Rather than risk alienating their hardcore fans, they maintained some artifacts of a bygone era. I'm not complaining about classes/levels, which I will talk about earlier. It's hard to put your finger on exactly what feels "wrong" however, since everyone seems to have their own pet peeve about the system. This is probably why they left most of these items in. I personally like the class/level system, but many people hate it. (And honestly, the d20 system can survive without it, see Call of Cthulhu's d20 conversion for an example of a classless system.) However, my own pet peeve is that you can't have higher skills without having higher levels. So, an apothecary needs a high level in order to be an expert on poisons from the realms. They have an "expert" class so that you can create these characters, but in order to give them a high knowledge skill, you need to give them a high level. And a high level translates into a high base attack. So, D&D forces a link between the "Good at picking a lock" and "Good at swinging a sword," even when that link exists only because of odd rules.
The class/level system is both a strength and a weakness, in itself. The benefit of classes is that it allows a quickly assembled team to be created. Dungeons and Dragons is a game of teamwork, unlike Vampire: the Masquerade or Amber Diceless Roleplaying Game which encourage individual goals. In those games which encourage teamwork but don't provide classes, players frequently invent classes. As an example, in Shadowrun my players will frequently ask each other who is playing the "decker", the "rigger", or "the mage" even though it's possible (but not wise) to make a character which could fit all of those tasks, just very poorly. Players will restrict themselves to certain archetypes, even if they're not forced into it. D&D allows for a surprising amount of flexibility within the classes, so that a 1st level fighter could either be an archer or a swordmaster, or even a master of the whip. But, there's no means by which you can create a character that my roommate wanted to desperately play: The mage who no one knows is a mage. In Ars Magica (a surprisingly generic system), you'd just let him take magic skills, and he'd accept a penalty for casting silently and without gestures. (In John Wick's diatribe against d20, he claimed that you would be unable to make an Orc Bard or a rich noble, which is entirely false in the new system. An orc bard can be played straight out of the PHB, and a rich noble would merely be a 1st level Aristocrat (an NPC class.)
The benefit of levels is that they make players happy. Players look forward to gaining new powers as game play progresses. The points system could achieve a similar goal, but few people use it in such a way. In the d20 system, a 1st level fighter gets a +1 to hit and a "feat." At second level, he knows he'll get another +1, and another feat. He knows that after 1000 experience (approximately 13.3 1st level encounters in a group of 4 players), he'll gain this benefit. In Shadowrun, my runners know that they'll earn approximately 3-5 karma once in a while, but they never really know what powers they'll be able to afford. They have their goals, maybe a +1 to a stat, maybe another point of the submachine gun skill, but the increase is so gradual that they frequently cannot look back and see an honest increase in power. On the other hand, after four months of playing, I honestly can tell that my cleric (who now multiclassed as a paladin) is tremendously more powerful. However, I am restricted in that my only form of specialization is by taking a new class to gain new powers and by spending my skill points according to my interests. This is very debateable weakness, however, since the class-level/feat/skill system allows for a greater than expected level of customization.
Another drawback is that there is a lack of a "merit and flaw"/"advantage and disadvantage" system. By that, I mean a system where you accept some sort of disadvantage (such as "one eye") and in return you can purchase other abilities, or even other advantages (such as "good sense of hearing.") Most other systems offer this, especially point-based-systems where they are an integral feature. I'm of a mixed opinion about these, however, as I notice it leads to many players creating color-blind amnesiatics, and other collections of trivial disadvantages. I can understand the compulsion to minmax, and in fact I respect it. However, I want to have my roleplayers (who don't spend time studying the latest method of minmax) to be on equal footing with my minmaxers. (One of the designers actually presented a seminar at GenCon for D&D which was "How to minmax your characer." I respect that, since by making the information publically available, it gives everyone equal footing and also opens up discussion about rules balance, just like Open Source Software, rules obscurity doesn't create security.) Still, I would like to have seen some way to reward someone who wanted to play a character with one eye, or find some way to allow a player to start out with double the starting cash, or as a member of an outcast family. I do not accept the standard WotC argument that this can be accomplished by Feats. Legend of the Five Rings is a system with Merits/Flaws, and it has a d20 System conversion in the Oriental Adventures (the third edition version) books. You can see the breakdown of the conversion in the method by which you have to basically throw away "character story" in the conversion process. (I'm sorry, there's no in-game benefit to the fact that your grandfather was a war hero, so your name is well known to The Unicorn Clan. I guess you'll just have to write it down on your sheet, I'll try and remember it.)
With some tweaks, GURPS or Hero System, or even Ars Magica, could easily suit my needs as a GM to tell a good story in any genre. I have a bag full of GURPS books in my car which I frequently use to design games that I will never run (GURPS is VERY good for running horror games). But, I run two D&D games a week and play in a third. In my own area, it's the strongest system available. When the third edition was released, both my 7th Sea and Shadowrun campaigns were dropped in favor of D&D. It's not that the system is that much stronger, it's just that right now we're all feeling a wave of nostalgia. When I open the books I can remember the smell of fall leaves in the air as I sat on my front porch and read my First Edition books, hoping that one of the other kids would ask me about the game. That's the most powerful force it has going for it right now. But, it's still refreshing that once I've become familiar with the "complex" combat rules (the combat rules are no more complex than any other system, it's just that most situations that CAN arise are covered in the base book), I no longer need to learn new ones. (Shadowrun still throws me for a loop with some of its "sub-games" such as Decking or Rigging. I can't even begin to describe how lost I was at trying to handle a car chase/combat.)
This node has caused a lot more discussion with people, which is a terrific thing. One thing that was mentioned by TheBooBooKitty was that he prefers the Hero System's "more realistic" take on injury. In Hero System, you get "hit" less as you gain levels/armor/what have you. Armor absorbs damage and better defense completely avoids it. The d20 System, as all systems are, is an abstraction. The concept of hitpoints is more than just capability to survive physical damage. An old Dragon magazine told me to think of hitpoints as being the capability of a fighter to shrug off damage, the ability of a rogue to duck to minimize the injury from a weapon, for a cleric to gather strength of faith to survive injuries, and for a mage to have woven secret enchantments to ward off incoming attacks. So, losing a hit point doesn't mean you took a "wound", rather you're that closer to dying from wounds. It's a moderately "bad" dodge, I agree, but I accept a certain level of abstraction in order speed gameplay. Now, with that said, there's nothing that says that the d20 System has to incorporate hit points. In fact, two published d20 systems by WotC already have a non-hitpoint based system for handling injuries: the d20 Call of Cthulhu and the d20 Star Wars (make sure to get the revised edition, not first printing.) If you don't want to go out and spend $40 on a new game, you can go out and pick up Dungeon Magazine 91 and get a game called "Shadowchasers" (by Wizards of the Coast) which also uses the Wounds/Vitality system. The crux of the system is that as you go up in level, you gain vitality but you never (usually) gain wounds. So, a critical hit with a blaster will even take out a 20th level Jedi, should everything go wrong. (Armor also doesn't make you harder to hit, it soaks damage, but that's a different argument.)
Also, in searching for more info on wounds/vitality, I turned up this article on Monte Cook's website. It details the "Good Things" about D&D, and I really respect his opinion as a designer. (He did design the Third Edition of D&D along with Skip Williams and Johnathan Tweet, two other people I hold in extremely high esteem). http://www.montecook.com/arch_lineos46.html