Simply - MW2 minus One Man Army and quick scoping.
Lane maps - all the boost jumping opened up the maps to many different ways to move around but going back to boots on the ground I would love to see the maps go back to the lane style maps. For me, getting shot in the back because of the 5 different places you can be seen from, is one of the most frustrating things.
Remove quick scoping - CoD for a long time has had quick scoping and there has been a few false starts of removing it but no-one has been brave enough to remove and it keep it out. I understand the need for some aim assist but if you want to fire a sniper you should have to fully scope in.
OHK nerf - Almost all snipers are OHK irrespective of where they hit you which needs removing in my opinion. Headshot and maybe torso hits should be OHK but no way should getting shot in the toe be a OHK.
Recoil - they just need to add some! The guns should not be laser accurate and there should be some skill in controlling the recoil.
Fix multiplayer latency issues - Up there with being shot in the back is being insta-killed. I think everyone has experience this at some point of walk round the corner and die before you can react yet the kill cam shows you walk round the corner, enemy aims, sprays around a bit and kills you. I don't think dedicated servers are the holy grail to fix all problems but it should improve things greatly. Even if they don't want to go that far then go back to MW2 style - yes the host had an advantage but everyone else was equal. I'd even accept tighter geographic limits - I used to play regularly with someone with a netduma so they could limit to only UK players and the difference was night and day.
Good kill streaks - it seems of recent years the kill streaks have seen nerfs. Presumably to not have them so OP that it stops being 'fun' and everyone quits. However the higher kills streaks should be OP! If you have managed to get a 12, 15, 20 etc. player kill streak then you are dominating the other team anyway so OP killstreaks wont change much.
The NUKE! - Nothing better than hitting that 25 player kill streak and calling in that nuke.
Claymores - I miss them I also miss scavenger resupplying them too so you could plant many of them.
That's all I can think of right now but I'm sure there are other things too.