I was on the DxDDP last May. Not being a big breakfast eater, I chose to do an early TS lunch everyday, then a 2 credit Signature Meal every night. I ate at Flying Fish, Artist Point, Yatchsman, California Grill, Jiko, Narcoosee, and Citricos. I was never even asked if I was on the DDP until the waiter handed me the menu. At the Flying Fish I had the calamari, scallops with risotto, and a cheese plate. I definitely ordered off the regular menu.
I go to WDW several times a year and have been using the DDP since the month it first came out. I have never had to order from a different menu. Now the first year it came out there were a few things not included at restaurants not owned by Disney. I know WGP Express had two items at that time that were not included and Planet Hollywood had one item. Those items have since been removed. Also I believe Marrakesh restaurant has two items where you have to pay a surchage, but again not a different menu. Also there are two sushi items at California Grill that you can't get as an appetizer on the DxDDP. I believe those are the only limits on it.