Well first of all, the beta wasn't too representative for the actual full game. Many things have changed and guns have been balanced and nerfed.
It's not that mw3 isn't good. I think it's just boring, I had fun playing mw2 for a couple of months, but after that it was boring, same with black ops.
And I already talked to people about it and it feels more like a expansion to mw2 than that it is a new full game. And that's just not worth buying for me, because I'm already bored with it.
But that's my meaning, people that aren't bored with mw2 and black ops will certainly love mw3.