Everyone forgetting Halo CE's story had some quality and very visceral (at the time) surprises in its own campaign. The shift from standard space marine versus alien soldiers to running for your life from the Flood was pulled off very well in the first game. This never came across as strongly again because in sequels you expect the Flood to show up whereas, in CE, players had no inkling that the game would change tone in such a way. Some nice sci-fi environments doesn't hurt either, even if the original Ringworld had more interesting concepts and designs, the Halos are still visually engaging in their own way.
Multiplayer/gameplay-wise, Halo was very nice for making an accessible console shooter. I also think it did vehicles and large open-area maps pretty good as well (I can't recall many other FPS games that pulled this off as nicely at the time).