As of this writing the most current version of the original Final Fantasy game appears on a compilation for the Game Boy Advance (Final Fantasy I & II Dawn of Souls). This version of the game fixes a lot of the problems that existed with the original game, however it creates a few new ones.
To start this version of the game replaces the graphics with better ones, it appears to be a direct sprite replacement job, as is often done with updates of this type. It looks pretty good and I honestly have no problems with the new graphics at all.
Final Fantasy has always been the odd one out of the series, it did things differently than the other games and had somewhat of a different feel. With this re-release it seems that they made an attempt to make this game fit better with the other ones, not to mention taking the opportunity to fix some of the most glaring problems in the game. The largest problem with the original game was that of characters making "ineffective" attacks against downed foes. That problem has been removed, as characters will now just attack the next monster down the line, not that your characters will be attacking most foes, but we will get into that later.
The original Final Fantasy game had a level system that didn't go that high, had no optional dungeons, and used pseudo-Vancian spellcasting (to be more precise it used a mechanic identical to the 3.5 Dungeons & Dragons sorcerer class). However later games in the series were well known for characters getting to truly pornographic levels (60, 70, the sky was the limit), and for using magic/mana points rather than spell slots, not to mention the optional dungeons with the super powerful bosses. So in an effort to fit in with the later series they altered the level system of the game to go up to around 100 or so. To tell you the truth I am not sure how high it goes, you certainly gain levels a lot quicker now, and no limit seems to be in sight when you end up beating the game. They changed the spell slot system to a mana point system, and added four massive optional dungeons. They also added some new items as well, and made a few other small changes in the combat system.
In expanding the level system they also severely cut down the amount of fights required to go from one level to the next, in fact they did it to a point where you simply didn't have to do any "leveling" at all, and if you bother to do so then you will completely remove the last vestiges of challenge from the game). Don't be surprised if you walk directly from the first town to fight Garland and end up at 4th level by the time you get there.
Now, when replacing a spell slot and level system (Vancian) with a mana point system you have to be very careful to maintain the balance. The first mistake every newbie does is create a system where the old spell slots are each worth their level in points, and a spell costs that many points to cast. That inevitably creates a system that allows the spellcaster to channel all their points into their best fireball spell, without ever having to be stuck casting those weak first level light spells. Needless to say Square basically did the same thing here. Now the other major problem with mana point systems, and the reason why I personally hate them is because converting magic into something that uses points quickly introduces the idea of a potion (usually blue) that fills those points back up. Having (common and cheap) items that fill your magic back up means that your black mage is NEVER going to run out of spells, even if he does nothing but cast his highest level spell every round. This really increases the power level of the spellcasters, who were previously dead weight kept around for one or two rather crucial abilities.
In addition to items to fill your mana back up, they also added items that can raise characters from the dead, and super duper healing potions that are actually worth using in combat, which means you don't have much reason to use a cleric anymore. There are also a few new weapons and some one use items that can raise a stat permanently. The stat items are pretty rare though and don't have much effect on the game.
The interface changes include some welcome ones, such as the ability to walk faster, buy multiple items at the same time and save whenever you want. However there are some changes to the inventory system that seem to be welcome at first, but serve to even further suck the challenge out of the game. In the past you were highly limited in the amount of weapon and armor items that you could carry around, in fact, each character could carry four weapons and four armor items. It wasn't all that unusual to have to drop weapons and armor (permanently) in the middle of a dungeon just to find out what is inside a chest (particularly irksome, since there is this Monty Haul area deep in the game that has a chest with a 1 GP cloak in it). Now the characters can each carry those four, plus the party can carry as many other unassigned items as they want. Another similar change is the fact that characters can now use other character's items in combat, by mid-game this means that every character is able to use about a dozen different spellcasting items at will.
The game also adds four optional megadungeons that have nearly unbeatable bosses in them. This would have been a great addition to the game if they hadn't totally dropped the ball on them. First off, they are huge, even the smallest one has more content than the entire original game did, and the final one has eighty levels to it. Unfortunately the random encounters (of which you will do thousands) in them are at a level that is too weak to challenge any party high enough level to get to the dungeon in the first place, and too weak to provide any significant experience points for said party. Have you ever walked around in the beginning of the game fighting imps when you are level 20? That is what this is like, just hundreds to thousands of fights without any challenge whatsoever. If you try the final one you might be literally 80th level repeatedly fighting foes more suitable to a 4th level party. True, there are some insanely tough megabosses scattered in these dungeons and some powerful items, but they aren't worth the monotony of spending dozens of hours of your life pressing the A button to fight monsters that aren't capable of damaging you.
Finally, with everything they did to fix (and ruin) the game, a few things stayed the same. Mainly the fact that all you need to do is grab four spellcasting items, give one to each character and use them all every round. That strategy takes care of all non-boss group style fights in the game in one round, two if you are unlucky. Non-boss single monster fights can never stand to a full round attack from all the characters, so there is no challenge there either.