For some great meat and potatoes try the Yachtsman Steak House, it's a short walk from the boardwalk. It's not casual but it is nice. Dress is resort casual, shorts are ok in hot weather.
A great casual place is the ESPN ZONE. This is burgers, fries, hot dog (best hot dog in Disney, possibly the whole state of Florida), chicken fingers etc. It is always active and all of the games are on the TV's. The main TV is a wall sized unit made up of about 25 TV's. They also have some live talk shows there.
Spoodles is casual, but a bit of a stretch for strickly plain food. It is a mediteranean type restaurant that serves Tapas. They do have good pizza and the atmosphere is good.
I'd stay away from the Big River brewery. It has ok burgers but the rest is poor. I'm sure that people will speak highly of Beaches and Cream. They have very good ice cream but mediocre food.
An often overlooked restaurant is Palio's at the Swan / Dolphin. Palio's is a very good italian restaurant in a very casual setting. Blue Zoo is not plain food, but very good. Shula's is a great steak house, but I prefer Yachtsman (why go to a chain when at Disney?). Enjoy. The Boardwalk area has many great options.