Serious Sam definately gave FPS gamers some SP things that most had been missing for a long time: lots of different sorts of enemies attacking at once, and large open areas. Their engine isn’t half bad either: not the highest polygon counts in the world, but lots of detailed textures and environmental effects (and that really awesome look metal morphing thingie).
Doom3, however, aims to completely re-invent the “closed-in” atmosphere with the use of real shadowing and very much slowed down gameplay. People who have played it have admitted to being really and truly scared by it.
I dunno: just trying to be fair since I’m a HL junkie, and haven’t really been an id fan from Quake2 on.