Bleach's biggest problem is that, when it decided it was going to make Aizen an impressively powerful villain, it overshot the mark by about a factor of 15. This made it so that in order for Ichigo to believably power up enough to take Aizen on, he would have to grind his way through several waves of bosses.
I mean, hell, it's been two whole arcs since Aizen beat Ichigo and it seems like he's still not even halfway strong enough.
There's a difference between "decisively stronger than the protagonist" and "should make the protagonist curl up in the fetal position and shit himself at the thought of ever meeting him again". Byakuya at the end of the Karakura town arc was the former. Aizen at the end of the SS arc was, and still is, the latter.