Balancing in MW2 is just plain horrible, the perks are especially guilty for this. The One-man army, danger close and noobtube combo is an example of this as well as Marathon, Lightweight and commando. If perks cannot be balanced at all, then they should make it so it's back to preset classes as opposed to this imbalanced class customization. It's a bit extreme to go back there, but even games like TF2 managed to be fun and balanced with preset classes.
The weapons has its ups and downs, it's not as unbalanced as the perks, but there are some glaring faults. Noobtubes are a given, the fact there's no consequence of having (As opposed to the disability of using any tier 1 perks) it equipped makes it a perfect attachment to get any free kills, just point in the general direction of the enemy and you get free kills practically, use One-army and danger close and you're set. The whole Akimbo "attachment" is also imbalanced, I'm sure you've seen Akimbo G18s and 1887s. As for the normal guns, most will opt to use ACR, M4, Scar, which really isn't that bad, but you see guns such as the F2000 and the FAL that are shafted aside and have made no effort to improve them so people would use them more.
Killstreaks are another problem, there wasn't any issue with the UAV/Airstrike/Helicopter combo back in COD4, because you can still deal with them. However, with the addition of AC130s, chopper gunners and other perks, it's just plain ridiculous on trying to counter them. The issue isn't the killstreak itself, it's more of the other issues that contribute to make it overpowered. For instance, the best anti-killstreak launcher is the stinger, at the very beginning, you get the AT4, which in my eyes is pure crap against killstreaks. You have to rank up to a certain level to get the stinger, and by that time, you've already played tons of games where you've tried to shoot down a killstreak but failed due to how underpowered the AT4 is. The other issue is the fact that no one bothers with trying to shoot down the killstreaks, but I'll leave that issue for now. The fact that these killstreaks exist, promotes players to go "lone wolf" and not work as a team. This of course, is a huge detrimental factor to why little to no teamwork exists in Modern Warfare 2.
In regards to the stinger, an essential item, being a weapon that needs to be unlocked at a higher level, I've come to the conclusion that essential unlockables such cold-blooded and the stinger really needs to given right when you start playing. Another solution is to just make it so everything's unlocked right off the bat, while this isn't a huge issue, there's no reason why any person that is a higher rank should have an advantage over someone else that is a lower rank.