I always remember hearing that Doom 3 was the black sheep of the family, and was more of a horror game than the rest. This really only holds true for the beginning portion of the game, and the rest of it was way more Doom like than I expected!
You just have to approach it with the mind of the Doomslayer.

My only real complaints are that the AI totally sucks, and will frequently spawn directly in your face to die immediately to a shotgun, and that all of the imp "Jump scares" are really annoying.

The weapons do feel lame at the start, but by the time you're in the midst of later combat, you totally forget about it.

The shotgun is also nowhere near as bad as everyone says, and it totally serves its purpose of making you run up to the enemies to kill them.