It is a different game though, it's more of a slow paced plodder with artificial gimmicks to induce tension. The flashlight implementation was an incredibly lazy design choice, I'm surprised to hear anyone defend it. The real problem with Doom 3 was that it should have been a more linear experience without things like monster closets, you can see they wanted to make something almost akin to Dead Space. They straddled the line with trying to accommodate fans and the traditional genre though. So they ended up with something in between that was disappointing to many.
This. Doom 3 tried very hard to be scary, whereas Doom 1 and 2 were more concerned with being good shooters. In D3, I don't think you ever fight more than 4 enemies at a time. In the previous games, you fought hordes of enemies. Serious Sam had more in common with Doom than Doom 3 did.