Business class gives one more legroom than coach seats on the same train, with the exception of the Palmetto, the Pennsylvanian, and (I think) the Coast Starlight.
On Northeast Regional and Carolinian, business class is 2x2 seating with more legroom than coach and window curtains.
On the Vermonter, Ethan Allen, Empire Service, Northeast Regional 65-66-67, the Lake Shore Limited, and I think the midwest corridor service, business class is 2x1 seating in one half of the cafe car. They also have window curtains.
I'm not sure what the seat configuration is like for the California services and the Cascades.
Business class gives you pre-assigned seating, which you can change to your preferences after buying the ticket. It also includes free non-alcoholic beverages.
You also get a better chance (but, of course, no guarantee) that the business class car will be less crowded than coach. This is one of the reasons why I selected it when I rode the Palmetto, even though the seating configuration isn't much different from the Amfleet 2 coaches. It may also be worthwhile on the Pennsylvanian, because not all of the coaches on the train are Amfleet 2 coaches, so one could be stuck in an Amfleet 1 coach with less legroom. That's also the main reason why I ride it on the Northeast Regional, plus on the Northeast Regional, the business class car is at the end of the train, so you also get the railfan window.
Coach class on the Acela is also marketed as "business class" for some reason. The legroom is comparable to business class on the Northeast Regional, the windows are larger, and there are curtains on the windows. If you're in Amtrak Guest Rewards, you get the same 25% point bonus that you get for business class on the other trains. However, free drinks are not included.