This is my opinion, not a true-flat-out-fact that-makes-me-hate Doom.
I always found Marathon more fun. I played Doom 1 and 2. The movement I found it most the time was slow, and the story was kind of shallow, but I did like the demons. It was like playing a slaughterhouse game, which is really fun to me sometimes when I'm overly happy.
Some things in Doom could have been fixed. Like, when I fought Satan, It wasn't some big floaty thing that you had to shoot a few missle packs at and had a lot of trouble aiming, therefore relied more on skill than ammunition. Fighting satan was like having an extremely powerful Imp that you had to like punch a few thousand times that freaking REGENERATED. It got annoying, and it could be fixed.
Doom 3's engine is pretty awesome. If they made a great story to go along with it I'd appreciate it a lot more. Or maybe if some generous fanboy imported Marathon to Doom3 it'd be pretty cool. Hmm.......