It sort of occured to me that if Aizen can show anyone anything he wants to, then more than likely what we've seen of his strength isn't real. Why wouldn't Aizen use that kind of ability to his own advantage in a fight and make the opponent think he was stronger, faster, or even in a completely different location than what they thought? While I don't think Aizen is weak, I doubt he is as strong as he appears at the moment.

Edit [Theory] going back to my theory that Aizen and Gin are the Grand Fischer hollow, we now know that Aizen can create illusions through hypnosis. The Grand Fischer hollow creates the illusion that its a little girl, and devours anyone who can see him/them.