I put in around 13/14 days on MW2, so I think I've got some experience with the game. To this day I don't feel there is a single perk or gun that is overpowered. Are knife runners annoying? Sometimes. Are quick scopers annoying? Sometimes. Are OneManArmy users annoying? Sometimes. Are noob tubers annoying? Sometimes.
But at the end of the day I don't believe any of those things are unbalanced, because I've never seen any one setup dominate the game consistently. I've personally found success using lots of different setups, but to this day I still don't feel there is one way to play the game, and I think that is because it's quite balanced.
I think quite often people think that if it annoys me, or this guy does really well with that setup, it must be because it's not balanced. I don't think that way. I think IW made the game that way, and they wanted there to be many ways to play that game. Believe it or not, I say this as someone who always plays the game as a straight up assault rifle/sub machine gun guy. I've actually never used any of the things I just mentioned, but I still feel they're valid ways to play this game.