I'll try to explain how my throught process went, or rather, my "emotion process". Maybe it'll explain it for some others as well.
When I saw Aizen decieve the soul society, I was like, "Shiiiiiiiiiiit". Sure, he was incredibly strong, that was like a bonus. However, I would like to ask fellow fans, weren't you more interested in hearing Aizen's explanations of things/hearing him talk more so than watching him swat down his enemies? I was always thinking I wonder what kind of mind **** he's going to pull next. You see, I believe as soon as Aizen was introduced as a great deciever, mind ****ery became synonymous with his name/character. I believe it had a lot to do with the way he was introduced, and even though I haven't watched an enourmous amount of anime, I haven't quite seen a villain being introduced like that. Myself, and several other of his fans perhaps, were waiting for his next big troll. Crush a few ants in the process? Awesome, but Aizen is the type of guy I patiently waited to hear talk.
He's also got a killer charisma in my opinion.
Then, you know what happened? He left the soul society and I/we waited very long just to get a few glimpses here and there. I think that anticipation led to my liking him more, he was introduced so strongly, and then just disappears for far too many episodes/chapters. He became a character of great mystery, I mean I bet so many of us wanted to see his bankai, but never got the chance. There's so much about Aizen that's still left to the imagination. And from what we got, I don't think he's a bad character at all, far from it. His defeat, the part about wanting to lose, it seemed like another apsect of Aizen's deception, something he kept hidden. I was dying for a background story, and I now may get it, so this too is why I'm excited that Aizen has returned.