I found it strange because it's common knowledge that Ichigo isn't the strongest character within the world of Bleach. Retsu has an overall score of 540, Yoruichi and Urahara are older than Aizen and were captains before him, and Yamamoto has been the Chief captain for a very long time. It just seems insane to me.

Much as I had innumerable problems with the everyone vs. Aizen fight, I was reasonably impressed with how they did the brief Yamamoto vs. Aizen brawl. Sure, there wasn't much direct fighting between them, but that's how Aizen works, and he sure as hell would have been extremely careful with Yamamoto. I was extremely disappointed with the fact that Yamamoto basically left him with no damage (that final kido strike might as well not have happened, for all the awesomeness it contained), but not for the fact that they didn't get into a direct blow for blow match, which even Aizen admitted he would lose.

As for Retsu, Urahara and Yoruichi, I think that's the result of knowing what Yamamoto's capable of. This is the one fight in the bunch that I wasn't surprised there was no intervention because Yamamoto is capable of so much AoE damage that he can't really control who he hurts. The fact that Retsu didn't fight him at all was certainly annoying, but that's a whole other story.

They did end up fighting for a bit, the Head Captain even got to use a high-level Kido on Aizen... XD