The game isn't very unbalanced when it comes to unlocks. Most of the firearms are well balanced like that. I could enjoy the RPD well into my 30's (level), where I switched to the AUG H-BAR which carries on the LMG line nicely. The SMGs and Assault rifles I also have no problem with, they all seem quite well balanced.
I only play Hardcore tho. As do I in BC2. Being able to withstand a full clip isn't apealing to me, I like the realism of only taking a few shots before you die. I also like how there is usually no map and crosshair, makes it a bit more interesting.
Things not balanced are the killstreak rewards and some of the perks. I get that doing well should be rewarded, but as has been pointed out by the OP, in MW2 you can do well, be rewarded, be rewarded by using your reward and then be rewarded again. So you just have the top players getting more and more tools, while the rest is stuck down bellow. I've had these reward streaks myself, I know how fun they can be, but strangely I think of the fun of other players too when I play. Getting raped over and over by flying things and stuff like that gets old after a while =p.
But what truly annoys me about the game is how the game has "become" due to players. You'll now have a tendancy to end up in games where you are expected to be a sniper and only do quick-scoping (which IMO is such a rubbish tactic). You'll actually have people call you a noob if you look trough the scope! In other words, if you act like a sniper, your a noob camper, if you use a sniper but run around like a retard thats ok. I've also been called noob for using Assault Rifles and LMGs.
Then there's the obvious exploits of perks, teleportation knifing and so on. I've already made a massive rant about knifing a while ago, so I'm not gona do another one. I'm sure if you search the forum you'll find my rant.
In the end, I think it's the small maps and low player count that ruin this game for me. The game becomes so unbelievably generic after a little while, especially when your like me and refuses to pay for addons like map packs which used to be not many years ago. (it's actually consoles that drove forth that trend, beore it's rarely if ever been any pay-for multiplayer expansions for FPSs, now they are all over the place).