There is no difference between the "FREE" Dinning Plan and the regular DDP.
As for restrictions, I heard of some surcharges at places like Planet Hollywood, and Wolfgang Puck's. In general you can order any app, entree, and Dessert on the Menu without a problem.
I know at some restaurants like Alfredos they have appetizers that are for 4 people and some that serve 2 and we were told they were not on the plan but I've heard you can share it and it be counted as two apps.
You can only order single serve items on the plan. You can still order stuff not covered by the plan if you want you would just have to pay out of pocket for the items.
I called WDW ADR regarding the surcharge of certain entree's. I was reaffirmed by the CM that there will be none. You could even order any non-alcoholic specialty drinks as well. So let's eat well and have fun!!