A great trip
Besides making the environments more involving and interesting, and developing a coherent path for the player through the game (you actually know WHERE you are at every stage) this game actually tacks on a plot that makes the whole game make much more sense. Maybe some people liked just walking through whichever door opened until they got to the end of the game, beat a boss, and knew they had beaten...something, but it's certainly preferable to know where you are, why you're fighting, and what will happen if you lose.
At the same time, there have been complaints that the game relies too much on "monster closets" and on scripted attack sequences. I couldn't disagree with this criticism more. Doom is one of the funniest games in involving it's own development staff in the game, and this technique is just another example of that: having the enemies pop out of closed chambers or spawn in just reminds gamers that they're ultimately fighting the game developers, not the devil. It's meta-humor.
Think about it, the whole plot of the game is...futuristic technology has opened the gateway to a demon-filled nightmare, and here's the game itself, based on some of the most high-tech technology ever seen in games up to that point. It's funny. Even when you discover that all of this has happened in the distant past, once before, the stone tablet you can see in the video is a reconstruction of the original Doom art.
What I do agree with is the fact that sometimes this game goes on too long. It tests your patience just to the point of almost breaking, before finally introducing you to a new type of environment. The pacing is a little odd, and sometimes shows a little too much of the old Doom "endless corridors" tendency, to the point of boredom. They could have tightened up the game somewhat.
Still, the thrill of blasting a hellknight with rockets, mowing down demons with one life-point left, searching for the next MedPack, or using the Soulcube against the Cyberdemon can't be beat in any FPS.