Well, lunch is QS so you order yourself at a "counter" (or before you arrive in WDW, online) and then seat yourself. A server DOES bring you your food though. You can pick which room you sit in provided there's seating available in said room. The menu is different from the dinner one, as you've probably seen, and you can only meet the Beast at dinner. It's pretty noisy at lunch and usually, people roam around to look at the different rooms and snap pictures.
For dinner you are shown to your table by a host/hostess and someone in your party is given "Lumiere" to carry to the table. At least they do this if there are children in your party; when DH and I ate there by ourselves we weren't given Lumiere lol, so not sure if that was a mistake or they only give him to kids! You don't choose your room for dinner and I've heard mostly reports that you can't request, although I've read a couple that you can. We've had dinner there 6x and never requested a specific room. Since dinner is TS you're served by a waiter/waitress like a regular restaurant; it's quieter and calmer although not quiet quiet, and the Ballroom, in my opinion, is still rather loud and cavernous feeling due to the size and the fact that some of the tables are close together. The Beast is "announced" every.......20 or 30 minutes during dinner? and walks through each room into his library where he meets guests; he doesn't sign autographs but you can have your picture taken with him. One other difference is that for lunch, you can get the "Master's Cupcake" which has the grey stuff frosting. For dinner, it's more of a grey stuff brownie-bar thing. Finally, I feel that people don't roam around as much during dinner. They still do to a point, but they don't seem to get as close to other diners while looking around as they do at lunch.
I think that's about it! We LOVE dinner and weren't that impressed with lunch but there are tons of people who prefer lunch.