You didn't play mw3 yet you called it 'horse shit'. I personally tend to make my judgements from first hand experiences. I even sat through a few episodes of Two and a Half Men just to make sure I knew it was horse shit.
A whole class of weapons is overpowered. Not one single weapon, a whole class. That's the whole point. In almost any single situation the smg's are a better choice of gun. Forget the obvious advantages of using a smg, like the ability to run faster. We're talking about ridiculous accuracy from the hip, at range. Huge bullet capacities. Ability to out shoot assault rifles with ease at range. That, coupled with the map design, makes even using an assault rifle trying to compete against a smg a futile task. If you are going to build tiny maps but still have the smg's be able to out-shoot assault rifles, make assault rifles be able to compete with smg's in close quarters situations.
Also, I think it's a bit tricky when you make the call that this is one of the more balanced weapons list in any CoD. Particularly when you didn't play the last installment. After some tweaking, the weapons system in that game completely kicked the shit out of this games. That, plus the fact that you, me, everybody have basically ignored one whole class of weapons in this game, does not make for a balanced weapon system.
Thankfully, Treyarch are seeing it for what it is, and have begun to nerf the smg's, starting with this last patch. edit: and buffing sway for assault rifles for long range engagements, making them more competitive.