Antics
Besides being one of the greatest musical performance acts in all of history, skilled at their craft in both composition and improvisation, these guys are also masters of delivery -- not only a great act, but a great show. They've historically and routinely incorporated special antics into their shows and stage presence, and have structured entire antic-y events.
See also:
Note that, often, the antics are something of a distraction. For example, yes, Mike and Trey are playing their instruments while bouncing on the trampolines, but pretty steadily; meanwhile, Page is wailing away. And some find Fishman's playing the most interesting during the It's Ice gliders, on which Mike and Trey slide but don't play a whole lot.
Thanks also to Megan "AboveTrees", Dan@ Am-dig, Brian Thompson 10/31/98, and Kyle Guslas 1/21/99; Ilene Wolfman 7/5/99; and JWe STONE 9-21-98.