Half-Life 2 is also remarkable in its inextricable link to Valve's Steam application. One can either buy the game at a store in the normal way, or one can buy it via Valve's much lauded and derided online content delivery system. The retail version of the game is little more than the large download (over a gigabyte) and the Steam client placed onto physical media. Even buying it from the store, and even when you just want to play the single-player game, you must still register a Steam account and log in to it from the computer you plan on playing the game from at least once.
There was a curious period immediately before the release of the game when Vivendi, the plastic disc publisher of the game, decided to delay the release for a time. Under the terms of their contract with Valve, they could do this for up to six months, and any online release over Steam had to take place at exactly the same moment as games would start being sold in stores. The date of November 16th was eventually decided upon, and all was well. A few people managed to buy early release copies of the game, but this did them not a whit of good. You see, people had already been allowed to download the game over Steam, and since the retail version is not really any different, neither version would be allowed to authenticate until the appointed time. When that time did roll around, the Steam authentication servers were rocked greatly from all fronts, but things generally went smoothly.
This online activation scheme has the obvious effect of making the game not work for those people who do not have home Internet access, but it can be argued that these people are not part of Valve's market. (In fact, such an argument is a truism: Since they cannot play the game, they aren't part of the market. QED.) However, as even simple dial-up access is sufficient to activate the retail copy of the game, the impact on sales should be minimal. Also, most of Valve's dedicated fanbase already have Steam accounts, as Valve made the ingenious move of switching the first Half-Life over to the service last summer and shutting off the non-Steam multiplayer server trackers. This forced the massive userbase of Counter-Strike and Half-Life's other assorted mods to use this new and largely experimental service, which had the double effect of acclimatizing people to the service, and of letting Valve work out the biggest kinks with a massive army of conscripted beta-testers before its big test (namely, the release of Half-Life 2).
So it was that the release of Half-Life 2 has gone remarkably well. There are a few issues that have cropped up, but (based on anecdotal evidence) these issues are limited to two or so specific problems, which bodes quite well for Valve releasing a patch or two to clear them up. The first of these problems is a crash issue: certain people have reported that the game crashes at a specific point (which seems to be different for each person) every time they try to play through it. The second is a mere slowdown issue, which is similar, but merely causes slowdown instead of a crash at those points. It's even possible that these are the same problem. (Update: And a patch has now been released which seems to have fixed it, a mere week or so after the release.)
HL2 has shown that Steam works, and that Valve knows what they are doing. More power to 'em.