I'm not nearly as harsh on the newer episodes of The Simpson's as most people are, but I will happily admit that there have been some absolutely terrible episodes in season 27. This, however, is not one of them. Barthood is probably one of the best episodes in the last five years. While there are some iffy plot points that don't match up with classic Simpson's, Bart's never before seen relationship with his grandfather, for example, that's OK. The Simpson's needs some change. That said, the Simpson's also needs tradition and this episode provides it.
Barthood is different from the other future episodes (the most notable of which probably being Lisa's Wedding) in that it doesn't focus all of its humor on the future. The fate of each of the characters is not particularly ingenious as one might expect from a comedy show, rather it is drawn form the same mix of realism and serendipity that made Boyhood so lifelike. In that way, though the episode does not exactly mirror Boyhood, it is a wonderful adaptation.
Then there's the humor. This is one of the funnier episodes from the last few seasons and it delivers with the same mix of wit, slapstick, and allusion that all of the best classic episodes do.
On the whole, Barthood is the blueprint for what a modern Simpson's episode should be. The story is touching and humorous like the Simpson's should be, but is also profoundly different from the episodes that came before it.