I'm pretty sure you get a discount on all merchandise in the parks (and I think Disney Resorts too?). You also get a discount on park tickets at Company D including discounted Water Park, DisneyQuest, etc. tickets. If it has to do with Disney and is not quick service food, you usually get a discount on it, e.g. sit-down reservations, tours, PhotoPass packages.
Around the holidays the discounts increase, usually. The one guide line you should follow when it comes to the CM discount is you usually do not get a discount ever on quick-service locations (e.g. the fast food places and the food stalls, although you may rarely get lucky with those) The only place I have EVER gotten a discount is Flame Tree BBQ in Animal Kingdom, but nowhere else that I've tried (popcorn stands, turkey legs, other quick-service places) ever offer it. If you're at a table service restaurant, though, chances are they give you 20%, but they tack on the 18% gratuity on top of that, so it's like your discount is paying for your tip.