Personally, out of the three Call of Duty games that I've played, I preferred how Black Ops felt. The guns may not have been very distinct between one another, but the ones that worked, worked well without being overpowered to the point of broken, like the UMP45 in Modern Warfare 2, with its LMG power, Sniper Rifle range, and SMG mobility.
Quickscoping is more than likely going to stay, simply due to its popularity with fans, and the fact that it makes Sniper Rifles more viable as a weapon class in constraints of Call of Duty's map size. But, I would make it an acquired skill, like it was in Black Ops, as opposed to how Modern Warfare took it.
Deathstreaks are going to have to go. Especially not anything that would give you, say, a chance to kill your would-be killer. That includes Martyrdom and Dead-Mans Hand. (Yes, I still say that Second Chance, in any incarnation, is utter bullshit.)
As for Killstreaks, they seem to be a fairly integral part of Call of Duty now, but yes, aerial Killstreaks, such as choppers or jets, need to be toned down so that they don't become something that will guarantee a match. Failing that, bolster anti-air measures, like lessening the time it takes for a lock-on, or make firing your primary able to do more damage to air support.
For guns, variety. Not just in the weapons themselves, but the attachments. Variety can make things a lot more interesting. Oh, and more importantly, balance. No one gun should excel over the others, even in its own class. All should hold their own, and all should have flaws and quirks.
And for Equipment, Tactical Insertion has to go. When its used properly, more often than not, it gives the opposing team an easy kill since its being camped. Otherwise, it promotes boosting.
And, last but certainly not least, fix the spawn system. Fixing that can easily avoid a good bit of player rage. Better yet, if possible make it so that players can neither spawn camp, or spawn-trap.