I'm only an hour or so into the game (Alpha Labs), so maybe the repetition argument is lost on me, but I'm really failing to see what the problem with the game is.
I don't think the gameplay is shallow or repetitive. First person shooters are my favorite genre, and I've played most of them, and Doom 3 gets a lot of things right almost all FPS's get wrong. First, the controls are spot on. No mouse lag or oddities in the mechanics. Weapon change times, movement and the level design is all fantastic.
Even simple things like the item pick-ups, doors opening, in-game GUI's, and the way that your character interacts with other characters (the seemless lowering weapons/flashlight to talk), it's all polished and well designed. Nothing in the game has frustrated me. Not even dropping to the desktop to go visit a URL to get a code for a locker.
Maybe it was gimmicky, but the chaingun was worth it =)
As far as the actual gameplay goes, I don't think it's shallow at all. I feel like I'm fighting, not mindlessly shooting. If you're finding the game dull, maybe you should be playing it on a higher difficulty, rather than criticising it? It's often said to fully appreciate Halo, you should play it on Legendary - so the strengths of the games design and AI come through.
Halo is doing a lot of things different to Doom (in the type of combat, AI and environments), but my experience in Doom 3 has been anything but dull.
It seems like Doom 3 is often criticised for failing to be something it never something it never promised to be.
