Halo was superb IMO! The aliens have superb AI, they toss plasma grenades at you if you charge at them or whenever they think they'd be able to stick it to ya (like if you are on a vehicle, turret etc). The enemies were fun to kill, the blood effects were cool, their bodies flying through the air and landing in positions to fit the environment (eg, if an alien falls on a ledge, instead of the body just laying on top of nothing as if there's an invisible floor, they actaully hung over the ledge. I even shot a grunt that was in front of a support beam, and he put his arms out and flopped against it

). The texture details were amazing (flashlight reflecting off metal, realistic looking grass, ice and water effects) and never blurred when you got close (unlike other games apart from Deus Ex). Instead, the detail would look better! Scratches of metal, bumpy tree bark, sand granuals etc.
Some levels were a bit repetitive, but I still found them fun. Outdoor levels were the best, however and looked magnificent (The Silent Cartographer is my favourite level). I also liked the flood, they were freaky and yet very fun to blast to pieces with the shotgun!
I really can't wait for Halo 2, even better visuals with the story of Master Chief continued... it'll be superb!