Everything on the menu...except add ons like Lobster Tails and of course, no alcohol. When you read the menus on here, you will see what every guest of WDW is handed (DDP, cash...)
When you arrive, you can always ask your waitstaff. The servers are more than happy to assist you. We were always given the best advice to optimize our credits.
On the regular DDP you can order any entree & dessert at a TS meal ( if it is a signature restaurant you would use 2 TS credits). Any apps or add-on sides you order are paid out of pocket.Just let your server know what plan you are on and they will let you know what is covered.
to the extent that most menus at WDW have been limited to accommodate the dining plan, yes. To the extent that there is one menu for DDP and one menu for everybody else, no.