While I didn't enjoy Keyes, I loved all the other levels. Environmentally, yes, they were repetitive, but the combat scenarios were so much better than those in Halo 2 or even Halo 3 that I hardly cared. I'd prefer if the game had been longer with more repeating environments, actually. You saw some of the same bridges or hallways but they were always littered with completely different combat scenarios. And the fact that all of this architecture was ALIEN meant that it made sense within the fiction.
I have played a TON of every Halo game, and none of them come close to the amount of hours I poured into Halo: CE, not because of the different level designs, but because of the combat balance, weapon balance, AI, mission structure, etc. The openness of the levels is probably the single most important aspect, it meant that once the AI was released, it was free to roam in different directions creating a more realistic sandbox with much less repetition. In Halo 2, I remember certain alleyways where a jackal would pop up in the same location and you could memorize spawnpoints and do the same thing everytime you played. It was like Call of Duty.