I realise I am alone in this, but the thing which annoys me most in MW2 is Stopping Power. I've lost countless duels with people who have a similiar gun to mine, even when I spotted them before they saw me, just because their bullets do a lot more damage. In past games there was Juggernaut which countered that, but for some reason they removed it, so that balance is now lost. I'd say that the fact that at least 80% of the players use Stopping Power is another good indication that it might be too powerful. However, none of the people I've asked agree with me, and noone has complained about Stopping Power in this thread either, so I guess I just si teh n00b.
The other main problem for me (and I am not alone in that one) is campers. People hiding in dark corners of rooms with invisibility perks, or lying prone on rooftops with a sniper rifle... ugh. Annoying as fuck. Almost makes me want to hang around my team's spawn area and just try to pick off any enemies who pass by, but I don't want to lower myself to their level. The only thing which provides some relief is the heartbeat sensor, so I use it often.
With Stopping Power, campers and the general asshattery and cheap tactics of people online I'm pretty sure I won't really be playing MW2 anymore a month or two from now. It's kind of a fun game, but most players care way too much about their K/D, and it shows.