My biggest problem with H4 is the lack of playlists. I miss all the CTF variants, team snipers, lone wolves (regicide is just dumb), king of the hill, action sack, team doubles, etc etc...
You should also utilize more of your maps in the playlists. Swat is complex, adrift, and abandon, and rare but sometimes solace. I have never even seen ravine in any playlists. Too embarrassed to use your own bad maps? Also the maps could be a lot better. In halo 3 whenever guardian showed up i got excited and competitive. Now my mind set in halo 4 is not "oh boy i hope i get -map name-!" it's more "i hope i don't get -map names-"
Also a H3 style ranking system would be nice. Halo 3's was very faulty but they had the right idea. they tried what no one else wanted to try and ranked people by actual skill, not experience points that any crummy player can earn. Perhaps your rank can be determined by points per game and k/d. I can speak for a lot of people when i say, people with good k/d's can have really crap w/l ratios simply because they get put on teams with crummy players. Before 50's where being boosted and sold, they were intimidating and hard to earn. I think a perfected, halo 3 style ranking system would greatly improve the game
In Halo 3 i would only stop playing because i felt i'd been playing too long, infact it's like that for most of my games. In Halo 4, it's the first game where i stop playing for the day because i'm honestly bored or too frustrated to keep playing.