1. Texture detail.
If you zoom in on just about any texture in Halo 1, you'll see a very fine level of near-photographic detail. This ultra-fine layer is almost completely missing in Halo 2.
2. Lighting effects.
Although Halo 2's engine is capable of a very nice bloom effect, you rarely see it outside of cutscenes, and it always causes a framerate drop. Halo 1 gives us a beautiful neon-like glow effect visible on the sniper rifle's night-vision scope, door locks, vehicle headlights, switches, and plasma shots/hit marks. You can also use the almost photorealistic flashlight at any time (compared to just dark areas in Halo 2).
Halo 1's is simple, atmospheric, well-paced, and both literally & figuratively ends with a bang. Halo 2's is complicated, bland, rushed, and unfinished. Halo 1 feels like a grand adventure, while Halo 2 merely feels like a bunch of random flashy setpieces.
4. Weapon balance.
Halo 1: Although the pistol is a bit overpowered, everything else has it's pros & cons.
Halo 2: The sword & carbine are incredibly overpowered, the shotgun & needler are worthless, the sentinel beam is a tacked on bit of flash, and let's not forget the n00b combo!
5. Artificial intelligence.
Halo 1 gives us hiding, flanking, and grouping, all well executed. Halo 2 has the same features, but they feel a lot cheaper and more random. The enemy leans more towards brute force than strategy.
What say you?