the problem with it would be that if you do it wow-style, 5 levels make a huge difference, more or less... so you would need seperate servers based on the level players have - either that, or the influence of levelling would be minor (as it is at the moment, with just alterations in perks that give you an edge, but no major advantage like infinite ammo hax would

). The closest thing to something like that, from what i've read, is probably MAG, where your role on the battlefield changes depending on your "level" and your character progresses in a certain task.
At the end of the day, what really makes those games is community... In modern warfare there are no dedicated servers you can pick so youll either have to stick with friends, or be put in a random server with random people providing mostly anonymosity and therefor an overload of jerks asuming that they wont meet you for a long time anyway, so the best way to vent theyr anger would be to call you or others a ......