My daughter almost always wears a princess dress into DL. I always carry a back pack so I bring shorts and a tee shirt just in case, but so far she has never been too hot or uncomfortable. She also wears them around town on random days, so she knows which ones are more comfortable.
Every trip she has had a huge fuss made over her by whatever charecter she is dressed up as. She has been picked to hold Minnie's hand to open toon town (wearing minnie dress and hat), been put in special seats, in short been treated like a princess. I have some great pictures from our various trips. She is shy too, but she loves the special attention she gets.
She is almost too old, I think this next trip will be the last time she wears a princess dress into DL. Oh and they wash really well, all of hers look great and have been washed many times.
In case you didn't know this there are park exclusive dresses that you can only buy in DL or DTD. The are expensive but every now and then one of those dresses is really fantastic. Most of the time they aren't nice enough to pay the price ($50 I think). We buy most of ours on special around halloween.