Black Ops was by far the most balanced Call of Duty game, we have all played. I don't care if you hate it, or you love it, we can all agree it was a balanced game. That's why Infinity Ward shoudl've implemented some of the new features Treyarch added into Black Ops.
1st Perk Appearances - I don't really know how to word it, so I'll explain it. Whatever perk you had in your Tier 1 slot (Blue), changed how your character looked. This was EXTREMELY helpful to me, and I'm sure to others aswell. For instance; Whenever someone had Flak Jacket on, I knew for sure not to try to kill him with any explosives. Or if they were using Ghost, I could see they were wearing a ghillie, and I knew that I'd have to lookout for him, because he would not appear on my Radar.
Weapon Customization - It seems with this whole "Level up your Weapon", and Weapon Proficiencies. IW was kind of on the same path as Treyarch, because they were all into this weapon customization, and making the weapon yours. But they took away a lot of the customixation for your weapon. You can't put your emblem on your gun. You can't put your Clan Tag on your gun. You have a very limited choice of reticles, and no Reticle Colour. No Lens colour. This doesn't really sound like a game changer, but it's these little features that make the game fun.
Special Weapons - I LOVED the Crossbow and Ballistic Knife in Black Ops. Sure these weapons weren't made to improve your performance but they were damn fun. They were also useful in situations if you used them properly. Crossbow was probably the most accurate explosive in the game, and the ballistic knife was really good for continuous knifing. Let alone being able to shoot a blade at someone!
Gunship - As far as I know, there's no Airial Killstreak in MW3, that you get to control completely. The Gunship in Black Ops was amazing, because of this. You could easily control your helicopter into the differnet enemy spawns. With predetermined courses for helicopters, it makes it really hard to not get shot down. Especially when you're in a random location, no where near the enemies, then getting shot down.
SR71 Blackbird - They kind of brought it back, but renamed it to the "Advanced UAV". It seems kind of pointless to me, because it's EXACTLY like the UAV, but shows the direction of the enemy. It's not that hard to see where the enemy is going, based on their spaawn, and where the objective is. The SR 71 was an amazing killstreak, because it was a real time UAV, and showed the direction for a full 45 second.
Emblem Customization - I really enjoyed being able to make my own creative emblems, and letting everyone see it. I know sometimes it was abused with people making inappropriate images, but that didn't really both me at all.
Combat Record - The CR in Black Ops was amazing. They literally almost had every stat you could think of, stored in there. I loved seeing my "Favourite Weapon", and other people's too. It really let you see everyone who was a great player, and people were not so good.
Search Preferences - Now I'm not to sure if they'll put this in MW3 or not, considering it was not in MW1, nor MW2. Search Preferences were really helpful for me in matchmaking. I live in Canada, and when I put mine to Locale Only, almost everytime I was getting a 3-4 bar connection to the host. It didn't work everytime, but it was still great to bring back from W@W
So what do you guys think? These are just ideas off the top of my head. What do you think they shoud've brought back?