As others have said, there are almost no restrictions on what you can order from the menu. Dining Plan Guests order off the same menu as cash-paying Guests. If something is listed as an entree, you can order it.
The only restriction that I know of are a couple of places that offer "add on" items, like adding a lobster tail to your entree for $xx more. Those are not included as part of your meal (but they'll gladly let you pay out-of-pocket for those items if you want them).
As for my own view of the Deluxe DDP, I did a whole spreadsheet last year. I was comparing DDP vs. Deluxe DDP vs. the Tables in Wonderland discount card (formerly Disney Dining Experience).
My results (based on how my friends and I would eat on the Plan), were that DDP beat out TiW only if the DDP was free.
And that if you upgraded the DDP to Deluxe, and then doubled-up credits to eat at a Signature restaurant almost every night, ate at a mix of Counter and Table Service for lunches, and used snack credits for breakfast items, it was an even-money upgrade. Meaning for the extra money paid into upgrading the Plan, we'd get back almost exactly the same value in additional food (appetizers, snacks, refillable mug). There were no real money savings involved in the upgrade.
-Rob