I think DOOM carries fantastic momentum as the it continues, more so than most games, as it only gets better as you continue (for the most part anyway). If the game was one generic combat encounter after another, it being 12 hours would have been more of a detriment.
VEGA is my favorite stage in terms of the buildup. Not only does each encounter get more intense, but it crescendos perfectly, requiring the player to use all the skills they learned and adapt to the increasingly difficult encounters the developers keep pushing.
The two Hell stages prior to it could have been consolidated however. Too many encounters in the middle of it that felt like it was just 'there' compared to how it started and ended, which were awesome by the way. With the beginning feeling like they finally took all restraints off on the combat in terms of opposition, and the end with the Hell Guards, which was a cool as boss fight.
