Of the minute (well, relatively speaking) pop culture parodies are not a "Simpsons" strong point -- the show's never had "South Park"-style quick turnaround, and has always been better at stories grounded in its own universe that use references as jokes rather than as a focus (to call out at oldie but goodie, consider the "Planet of the Apes" musical back in the seventh season).
And that Limbo is great, an opportunity for "The Simpsons" to play around with surrealist takes on its own characters and universe (something it did most memorably in "The Mysterious Voyage of Our Homer" back in season eight, with Homer wandering off into the desert to hallucinate after eating Guatemalan insanity peppers).
It's a skyline comprised of stacks of giant donuts and Duff beer cans. There's a Moe's Tavern (staffed by a Moe) on every corner. The Kwik-E-Mart is also oversized and done up like a factory belching rainbow-colored Squishees into a waste pool in the back. And to take Homer's description (and "Inception"'s seriousness) to its most ridiculous end, there's a roller coaster that zips around and through a massive version of Homer's head.
The first dream-within-a-dream finds the family back in their more jagged "Tracey Ullman Show" days, as they wait for family therapist B.F. Sherwood to see them. Homer demands a baloney sandwich, Bart makes faces at Lisa and everyone gets into a brawl. "The Simpsons" had its origins in being a straightforward goof on a dysfunctional family, and it has grown so much richer and stranger than that that it actually takes a glance back to its old days to remember just how much its changed. The show has become its own pop culture reference, and that scene in the therapist's waiting room looks just as striking, in its own way, as a bobble-headed Apu offering to give you your change in bacon.