In our two trips to WDW, we've eaten at:
Flying Fish, Spoodles, Microbrewery (I forget the name), and ESPNZone at Boardwalk Inn
Le Cellier and Japanese Sushi lounge at Epcot
Brown Derby and Sci-Fi at MGM
Whispering Canyon at Wilderness Lodge
We LOVED Flying Fish, but it will cost two meal credits on the meal plan. That said, a good meal there would easily cost you $50 per person, so it's still worth using two credits. My wife and I will be while we're there for a week over Thanksgiving.
Spoodles is also good, but I'm not sure I would go out of my way to eat there. We're staying at the Boardwalk, so we're definitely planning on going back. They have a variety of good appetizers or small plates that we ordered almost like a tapas restaurant, but they also have good regular entrees. It's also a large room, which means it's easier to get in on short notice.
I found the microbrewery on the boardwalk to be just OK. The food was pretty much your typical pub fare and the microbrews actually weren't that great in my opinion, and I do love microbrews.
ESPNZone is what you would expect - no more, no less. I'm kind of a sports nut so we stopped in there a few times after the parks closed to catch the end of that night's Red Sox game.
Le Cellier is a nice steakhouse with food that could be considered a little pricey depending on your budget, but the food is worth the price, unlike some restaurants. It's a good pre-Illuminations spot. Since you're on the plan it's a good spot to get "value" out of the plan, since a good dinner would likely run you $35-$45.
We found the sushi lounge in Japan to be a little limited in its selection, and wouldn't really recommend it for sushi.
Brown Derby is good. We've gotten the Fantasmic Dinner package the past two times we've gone, but decided against it this time, more for variety's sake than anything against the restaurant. I believe you can get that package on the plan, and it normally costs around $35-$40. We had no complaints about the service there. It was our first anniversary, and my wife was able to call ahead and get them to make a replica of our wedding cake top, since we obviously couldn't cart it down for the trip, (it probably tasted a little fresher too).
Sci-Fi is a good sit down lunch at MGM, more for the atmosphere than the food really. We usually go for lunch and sit in a car and order burgers and milkshakes and feel like we're in the '50's.
Whispering Canyon is probably great for families with kids, but likely an annoyance to any adults without kids. When we were there they had a couple of annoying rituals, like not stocking every table with ketchup, so that if you needed it you were supposed to just yell out "Ketchup!" and another table was supposed to bring theirs over to you. They did some other kid friendly interaction with the diners, which I'm sure was great fun for kids, but not for someone just looking for some food.