Well lunch is quick service and dinner is table service so there are a lot of difference. The way it currently is at lunch (and presumably will stay the same after the ADRs are implemented) is that you can either pre-order your food or order at the counter. You will enter the restaurant and sit in whichever room you like. A CM will deliver the food to your table (they'll locate you by Magic Band) and you will get your own drinks, silverware, etc from the drink station.
Dinner is table service, so you'll be seated by a host/hostess, have a server who will take your order and deliver your food, clear your plates, get your drinks, etc. The same as any other TS. Also different menu, and the opportunity to meet the Beast.
We prefer lunch because we like the lunch food options better and the ability to sit wherever we want. Dinner can be more relaxing and time consuming with a server, etc.
When lunch switches to ADR, it's speculated that everything will be the same as lunch currently is - just that it will be with an ADR rather than a FP (which is how you currently reserve it)