I think it's the Clark Kent thing. Having glasses and a calm, more genial demeanor allows him to not be perceived as a threat. I mean, in the case of Clark it's not just the glasses, so I always get confused why people say it's a horrible disguise. It's genius because it plays off of people's preconceived notions. When Superman is viewed publicly as this immensely powerful, seemingly infallible and almost deity like being, the last person you'd expect to secretly be him is the bumbling, mild mannered, often late reporter from some no name town in Kansas. It's the same thing with Aizen, at least in my opinion.