In plenty of previous posts, I've gushed about how Steam makes getting games painless and how cool the service is, but here's an article with the opposing viewpoint from some guy called Randy Pitchford.
His big thing seems to be that he simply doesn't trust Valve and that he thinks currently Steam is taking an unfair percentage of the game sale.
Which might be true.
But I tell you, I probably never would have bought CoD4 or Bioshock if they hadn't been on Steam and playable in a few hours. I know for a fact that I never would have jacked around with Titan Quest or the Oblivion games if they weren't on Steam and sold as cheap bundles as I wasn't about to drive out to Best Buy and go sifting through the bargain bins just to find copies of them.
I think the power of their distribution system is how easy it for games to keep selling beyond the initial launch window.