I've now played Doom1 and right after Doom3 to get the differences...
(Except the nostalgic thrill with doom1) for me doom1 has a plain/clear level design - the priority is on the enemies and - as Firoball mentioned - the "static enemies".
Doom3 (for example) seems to emphasize on "ambient" (darkness/complex leveldesign) what turns the focus - I like the "clean/predictable" gameplay of doom1 better....