Short answer: It's buggy. Very buggy. Some might say it's at a pre-release level of buggy actually, but that's be a cruel and heartless thing to say. :D
Longer answer: Steam as a concept has some merit, and there's some really nice ideas but the implementation has been awful. For a start...
Doom 3 (and Half Life 2 for that matter, though possibly not to the same degree) is that rarest of rare things, an "event game". Doom was many people's first introduction to gaming on the PC, and for that matter, to the world of FPS in general and it just blew everyone away that played it. While...