I don't really understand game companies. They say a lot about copying Blizzard, but then they never do. There has been one constant about Blizzard games ever since the first Warcraft: Odds are the game will run on your machine.
And this is crucial. If your machine can't run the game, why would you buy it? Blizzard constantly sets the system requirements low. They make their games available for Macs. They deliberately stylize their art so that it still looks good on low end machines.
Warcraft, Diablo, Starcraft. Time and time again, Blizzard pulls the same trick, and no one else seems to learn.
And yet other companies do not seem to realize this at all. If you can't run the game, you won't buy the game. I play on a laptop, with a built-in video card. No nVidia, no ATi. And yet WoW runs, looks halfway decent, and thus I give Blizzard my money. I'm not going to go spend a couple thousand dollars just to play your game.
It's not the only reason Blizzard is successful, but I think it's a major one.