When you go to the store to buy a game, you're expecting to buy a tangible product -- the game cds, and maybe the cdkey that comes with them. As long as you have that CD, you'd expect to be able to play the game.
The problem is that when you go to the store to buy Half-Life 2 you aren't really buying a tangible good anymore, you're buying a license. Sure, it comes with CDs, but they don't actually contain a complete working game -- HL2 won't work until the Steam servers unlock it. The difference is that the game is more or less virtual now -- you've purchased the right to play the game, but you haven't purchased the game data itself. That resides on a server somewhere, and your access to that data is subject to the approval of Valve, and the wellbeing of their Steam servers.
And it gets even worse. Essentially now, you're buying a license, but you aren't even allowed to see the terms of the license until you've unpackaged the game. So what do you do if you don't accept the terms of that license? Most stores won't allow you to return opened software. The only choice you really have is to accept whatever terms they dictate, or eat a $55 loss.
Not to mention that this kills the longevity of the game. If I want to go back and play Quake or Doom, I still can. But what happens when Valve goes out of business and shuts down Steam? The CDs you bought become utterly worthless (and if you bought the game over steam, it's even worse, although at least you got to see the license agreement before you bought it).
Further, this adds a lot of unneccesary complexity. If I want to play a console game, I grab it, put it in the machine, and play. If I want to play a PC game, I have to free up hard drive space, type in a 1000 digit CD key, and then wait an hour while it installs 5 GB of junk. And after all that, I still have to keep the CD around. Valve has just made this process that much worse, because now I have to do all that, *and* wait for the over-loaded steam servers to unlock my game, and then update it, and then each time I want to play I have to wait for it to authenticate me. No wonder people are moving away from the PC as a gaming platform. It sucks.