Doom is still THE game. QuakeWorld players, and indeed hardcore fans of every game will tell you they, and the game they play is the best. But Doom was the first, first person shooter to break into big markets. It is the game that made iD software what it is today, and without it, we would never have seen a Quake series. Doom was the first game with real, playable network code. I actually was playing doom 2 at a lan a few weeks ago, and it was more fun that most games that gets thrown at shelves these days.
Doom has its hardcore fans, just like every game. People who played it, people who helped make TC's for it, or indeed people who still play with the source code. Doomworld has an interview with some of those people about what they think, Doom level designers, webmasters, and interestingly Doom co-creator and co-author John Romero.
They speak about what pitfalls iD should avoid, what the game could be like and just handy hints that could be taken into account in making the game. I admit to being a cynic, but its funny reading Romero's comments as they are all basically to follow directly on from his Doom, and not to change much at all, for example on the subject of the music in the game, "I would stay away from rock and industrial. I would keep it very moody and more like a movie soundtrack, with a nice variety of mood types, just like the original DOOM". Its an interesting read on what will be a constant topic of conversation for the next year or so until we see the game.