1. With maps, I don't think in general that they should all be bigger, maybe 1 or 2 maps like the maps from homefront or fuel which are ridiculously big, but also small maps which are good for 1v1s (shipment and rust). What made mw2 maps so superior to black ops maps I think was that mw2 maps had huge amounts of levels. maps like highrise, rust and favela were so awesome for their levels. The only map like that on black ops is kowloon. I would also like to see a map which is set entirely in a building which has several floors, like many of the maps in cs:s.
2. Stackable killstreaks, this made the game fast paced and fun to play. Some people say that this made everyone camp, but in mw2, to get a 11 killstreak, all you had to do was get a preditor missile. in blackops, you have to sit in a corner for 5 minutes or so.
3. Good perk system. Taking out stopping power was a horrible idea for black ops, it ment that there was only one good perk for the 1st tier - ghost. ghost, a campers best friend, slowed down the game as spy planes became useless.
4. quickscoping. this gave mw2 and cod4 a replay factor, the reason why 100k people still play mw2 each day. also, quickscoping made the cod community on youtube etc explode, and is one of the reasons why cod is the biggest game of all time.
5. Better spawn system. I know that a good spawn system is painful to code, but homefront got it exactly right. I've played that game pretty much since it came out and not once have I been killed straightaway from spawning. If a lesser game can have such a good spawn system, I'm sure mw3 can.
these are just my ideas.