The only one I've heard of is Emeril's. They don't allow tank tops or flip flops. The park can ask you to wear something else if your shirt has something offensive on the front.
Actually, I've never seen anyone stopped for what they were wearing. Well, take that back. I saw two guys a while ago dressed like they stepped out of "The Matrix" (long coats, black clothes) and security stopped them for something. I just hope they didn't ride Popeye!
All of the Citywalk restaurants (excluding Emrils) are very casual. The only restaurant I dressed up a little bit more for was the Palm but even that wasn't really necessary.