It has been 6 years since we last visited WDW. On our last visit, we had the Dining Plus. Correct me if I'm wrong, but didn't we used to get an appetizer with our meal. Now, I see you do not. Maybe I'm wrong.
In 2007 when we first tried the DDP yes you got and appetizer and the tip was included. In 2008 no appetizer and the tip was no long included but they also started the Deluxe and the Quick service and I believe you can also get a platinum that includes a bottle of wine and more. I have also read that Disney has added a few new restaurants to the plan for 2014, and some meals at select locations are no longer available for a DDP credit. See the Disney Food Blog http://www.disneyfoodblog.com/2013/07/11/news-2014-disney-dining-plan-changes/
Hope that is some help.
The DP has changed a lot since your last trip. Our first trip was back in 2007, and it was substantially cheaper and included more. We avoid the DP these days because it's usually cheaper for us to pay out of pocket.