And although it's not official, the California Grill manager I spoke with said there have been issues with skirts on women that were far too short.
It's a "requested" dress code of resort casual. Not required. The 5 things already mentioned by PP's are the only ones any CM will say anything about.
At any Signature Dining Restaurant, the table next to yours may have a party dressed in shorts, t-shirts and gym shoes who came directly from a day of touring the park(s).