A) Keep your shotgun handy and stay "safe" knowing you can shoot zombie heads off.
B) Have your flashlight ready and see whats ahead while not being able to do jack squat when a demon starts throwing fireballs at your face.
Many times I had my flashlight handy and something jumped at me and I frantically switched to a better gun while flailing around with my flashlight trying to stay alive.
Doom 3 also had a really great art style. I don't look at screen shots or boot it up and think its a dated game. It still looks great.
It also had some really cool stuff included like Super Turbo Turkey Puncher 3D and the whole martianbuddy.com locker.
I also really enjoyed the audio and video discs you could find all over Mars base and I listened to them all and thought they really added to the game depth. It was fun to find a new audio diary, flick it on and listen to the audio as I made my way through corpse strewn hallways.