Probably the Emblem Editor and the sheer level of customisation that Treyarch added with Black Ops.
That was awesome, in my opinion. Sure, the Face Paint was pretty pointless, but the fact that you could add your emblem, clan tag, custom reticle, etc. was pretty awesome.
I thought that customisation would come back bigger than ever with Modern Warfare 3, but unfortunately, it looks like I was wrong.
"Blackbird is back as an advanced UAV" implies that it's the exact same thing, but with a different name. As you've already been told, Advanced UAV is NOT real time, which is a big deal, AUAV CAN be shot down, also a big deal. Lastly, one thing you weren't told, is that Advanced UAV is countered by Assassin, where Blackbird could see you whether or not you were running Ghost.
The emblem on a weapon was definitely beast. It's been mentioned, but dolphin diving was pretty cool too. Some of the maps for sure. Stadium, Firing Range, Summit.
As is always the case, no matter what the game, there are always people that like to *****. Let's face it, $60 for a game that many have 10 days+ of actual game-time and people *****? Haha, that's pretty ridiculous. Let's see what kind of ranting we see in MW3.