Black Ops plays slightly different, but in terms of how similar it is to MW2, it is pretty similar with how it plays. Ye they've changed things but you won't notice particularly much difference when you play. I also don't really get why people say things like "it's just the same as the last one", "nothing is different", "why waste money". One way to look at it is, when you buy say the 4/5 map packs that come out, it might cost around £40-£50 to get more maps. A new game is basically a big map pack. The game is Call of Duty, if they change the game too much it's just not going to be the same game. The campain is obviously totally different and different guns ,challanges, perks, killstreaks etc in mulitplayer always makes a nice change. You've also got different game modes like kill confirmed (which is basically all I play) which is brilliant and the new 2 player game mode (similar to zombies but without zombies, explaining it briefly).
In terms of mods and hacks, maybe security is tighter in this game for the future. Treyarch done an extremely good job stopping it and it was always their aim to stop it and they succeeded, so Activision might have taken a leaf out of their book.
But the difference between Treyarch games and Activision games is quite a lot but still keeping it close enough to be a COD game. So really there's 2 years between each type of game. I really think people who moan about COD in that type of way arn't really thinking about what they're saying, they literally can't change the game too much, and besides, if they did, I don't think it would be any better in terms of gameplay, movement and controlls. They're all on a massive bandwagon. Wish they'd just say they don't like the franchise instead of moaning about it and buying every one and saying the same thing every time, if they don't like it, fine, move on don't continue to buy and play it.
I get annoyed plenty of times, but I still love COD, and I haven't played another FPS game that rivals it in terms of enjoyment. And I suppose that's why it's the biggest game franchise going.
As you also said about "fun and annoyances" yes you will, you always will have both of them, but you'll still enjoy it like you did on MW2 and maybe more.