A) The level design leaves something to be desired. It's not nearly as bland as Halo in that regard, but it could use some variety.
B) The game is scary as hell for the first few hours, and then becomes predictable. You might get a cheap scare every once in a while from something being right in your face when you open a door, but otherwise it's not all that frightening.
C) Not being able to hold a flashlight and a gun at the same time is understandable for gameplay reasons, but it doesn't make any damn sense. If they have the technology to create portals to hell, they should be able to manage to duct-tape a flashlight to a gun.
D) Despite the amazing graphical technology behind Doom 3, the physics stink. Few things are interactive or destructible, and they all seem to weigh about as much as a piece of styrofoam. They float and bounce around like they're weightless.
b) Play at night with lights turned off and with the volume up and with 5.1 surround (if you don't already)
c) Dude, it's just a matter of configuring your keys properly. Since Doom 3 has no alt-fire I have the flash-light on my right-mouse-button so you can jump between the flash light and your gun in a fraction of a second.
d) All I care about is ragdoll stuff and Doom 3 does that very well. Sure, kicking around shit isn't very well done but who needs that anyway. Although I have to admit that how Max Payne 2 used the Havoc engine was pretty impressive.
I play @ 1280x1024 on "high" which is neat and at that resolution, you don't need anti-aliasing (which probably would slow it down too much anyway).
A tip for everbody: if Doom 3 performs poorly on your PC and you want to update your PC, go for a better 3D card rather than a faster CPU. A friend of mine has an AMD Athlon 3000+ with 1 gig of RAM and a Radeon 9000 and the game ran at around 18 FPS @ 640x480 with medium quality and was absolutely unplayable at anything highter than 640x480. I told him to update his 3D card, he bought a 9600XT and the game now runs @ 45 FPS on 1024x768 with "high" quality.
I'm pretty sure Doom 3 will run fine on any ~2 Ghz PC with 512 MB RAM provided that you have a good 3D card.