I played through Doom 3 and generally liked the game. However, there were several things that did bug me though. Like everyone has said, picking up weapons or entering an invisible trigger box is lame. Those kind of monster triggers have been around forever. Ok, here's a to do list just in case John Carmack is reading this thread.
1. Make monsters quieter and faster... (Barons of hell were cool, but slower than fat people in a buffet line. When monsters make a ton of noise even when un-triggered, it takes away the element of surprise)
2. More baddies with non-dissolving bodies... (I want to admire my kills and then play with their remains.)
3. No more invinsible Bosses that require some retarded trick to defeat. (dying 50 times or consulting online hints is annoying. Taking down the Cyber demon with a 100 double barrel shots was very satisfying in the old doom)
4. More Monster on Monster action (Not only a strategy, but fun to watch. Remember the DOOM2 boss room, monster orgy??)
5. Pretend that Arnold had already been to Mars and oxyginated the damn planet.