This is a great one. Kind of to Wes Anderson's filmography what Corpse Bride is to Tim Burton's: the place where he steps outside of his own medium only to find the ultimate expression of his own particular filmmaking concerns. As many reviewers have noted, all of Anderson's Andersonisms feel natural here in a way that (many people feel) they don't in his live-action films. And his themes - the underappreciated son, the paterfamilias who won't grow up, the materfamilias whose role mainly seems to be to try to make him grow up - are all present and accounted for.
As is his care with music. Others will remark on the strangely appropriate Beach Boys placements; I want to point out what's playing faintly in the background of that idyllic scene the Foxes spend in the front yard of their new tree house, when Mrs. Fox is painting, Mr. Fox is reading his newspaper, and the boys are learning how to dive. It's "Love," from the 1973 Disney Robin Hood - in which, of course, Robin and Marian are foxes. Nice touch.