I don't like the plot all that much, it's farily straightforward and not at all suprising, and that goes for both games. On the other hand, I was never really woved by an FPS story(unless you count Thief: Dark Project) and Halo gets back in ambience what it loses in story.
Co-op is the mother and father of blasty-blast. We played Halo 1 like two years straight, trying to do everything possible inside the game. Which wasn't a whole lot, but "rocket ghost"(rockets+invisibility+rat race+one on one slayer) was very nifty. When the sequel game, we bought it the moment it hit the stores and played like 9 hours straight. Gravemind is still an awesome level. Even on Mythic-difficulty.
Ah, the scarab. I remember getting stepped on when the thing walked over the characters near the last engagement. Good times, good times. Last time I was so fed up with the whole thing I took a ghost and drive it on top of the thing, while it was moving. :D