Halo CE is awesome. AotCR is one of the best levels in the franchise, and one of the best levels ever. The Library is probably longer than it needs to be, but it is actually quite a fun level filled with non-stop action combat, a cool homage to old school FPS games. And Two Betrayals, with it's great atmosphere and those crazy three way battles is very good too.
And yes, the visual repetition is really the only "big" problem the Halo CE Campaign has, and it is only annoying during the first playthrough. Once you get into the combat and start replaying levels, it stops being a problem. The encounters are so fun and replayable that you don't really think about the repetitive textures and re-used geometry.
Never thought I'd see someone say that Halo 2's AI is better than Halo CE's. LOL.
Halo 2's enemies may be more dangerous, but only because they do more damage. The actual AI behaviour is way inferior to Halo CE, which still has some of the best AI ever seen in a videogame (IMO only FEAR can compare)
And yes, you can throw a bunch of grunts and jackals and a couple of elites in a room, and it will make for a way more fun encounter than anything found in Halo 2. The AI and the gameplay are just that good. As I said, the combat in Halo CE is still unmatched to this day.