"Maybe it's because I was raised on classical music concerts and operas, but I think going to see a performance (other than something like a rock concert) in shorts and a T-shirt is kind of rude. I don't go completely formal for a Broadway show, but I'll dress nicely- it's a sign of respect."
Plum, I wasn't raised on clasical music concerts or opera, but I agree with what you say. I always like to dress up a bit -- not full fancy dress, but a nice skirt/top or dress -- when I go to the theatre because I want to, and I think it shows respect. I do like to be comfortable too, but most of my nice clothes are pretty comfortable.
This topic always has a wide variety of responses.