I am a Half Life fanboy. Spent a lot of time in #half-life back in early '97, when I was supposed to be doing college applications. But Doom 3 is the one interesting me more now. I'm in the mood for something simple and oozing with atmosphere. Games simply don't have atmosphere anymore, and those that do are often games that need time to learn.
Doom 3 looks like it's all your basic skills. Slowly walking through dark corridors, blowing the hell out of anything that pops up, and having white-knuckle gunfights as you run for any safety you can, hoping the halls behind you don't end before you're caught.
Half Life 2 looks amazing, but I want pure a pure and simple mix of fear and action. I can't remember a game giving that in the last 6 years. Maybe it's more a reflection on me, but I'm hoping D3 gives a major kick.
Of course, I'll buy HL2 the day it comes out as well, and struggle to deal with both of them combined with the first year of law school, and probably flunk out. Go me.