Big, open maps. And just better maps than Reach. I love BTB and Objective gametypes like CTF on big maps. And Halo 3 did that perfectly imo.
Maps like Sandtrap, Avalanche and Valhalla are great for this kind of stuff and really takes advantage of the vehicles without making you feel like you need a vehicle to get around. Overall, the maps in 3 are just plain better than Reachs maps. Hopefully we see a varied amount of maps this time round. Big, small and medium.