The whole secret of Doom was NOT taking on super-tough enemies, there weren't actually that many in the original doom, the secret was one man walking into a room packed with Zombies, Imps and Demons, and walking out of a bloodbath. Doom 3 took itself way too seriously, it's like someone sat down and tried to think up 'how could this be feasible in reality?', which is totally irrelevant.
Basically, they tried to turn Doom 3 into a horror, which DID work, but I don't want to feel like I'm trying to survive, I enjoyed that feeling of 'Green hat? Check. Shotgun? Check. Hordes of Satan? Check. Let's go kick some ass to some Midi Pantera."