Textures and graphics actually fitted together whereas in halo 3 everything is way way way too shiny and the vehicles and weapons look out of place against the actual map
Sounds on guns, vehicles and basically everything were better
No jaggies
Storyline is better -
No corniness
can actually feel something for the characters
playing from the covenant point of view was awesome
at some points knowing you were meters away from the person you were playing in the last level was great fun.
When playing through the game again and again you would notice things which you never did before (dosnt happen in halo 3)
actually had some freedom running around in campaign whereas in 3 you are forced upon a specific path
voice acting MUCH better than halo 3
legendary was actually a challenge
fighting elites for the first half was fun
the flood wernt nerfed
Multiplayer:
Gametypes were actually fun
Assault wasnt ruined
territories was actually fun (none of this offence and defence bull)
neutral flag was actually in the game (isnt in halo 3)
1 flag was used more often
levelling system was better
no exp made the game less competitive in unranked playlists
maps in halo 2 were better
OH AND MOST OF ALL: The game wasnt ruined by anticipation
There is nothing I actually prefer in the third game to the second
Ill add more when I think of it...