I didn't hate it, but it felt very paint by numbers.
-Confirmation that heaven is fixed, and that Cas isn't consigned to the Empty forever! Yay!
-The impala in heaven with the original plates, especially juxtaposed with Sam on Earth with the version with the newer plates.
-The use of Brothers in Arms. How did I never think of how perfect that would be for this show?
-Dean's "I love that song," and the placement of "Carry on My Wayward Son" more generally.
-Sam's son having the tattoo, which also leaves a little bit of ambiguity about how far Sam distanced himself from hunting.
-Dean's joy in the pies, and the pie in the face gag. Just nice to see them having some fun.
-Dean telling Sam he looked up to him, and that Sam is stronger. The latter was just unnecessary--I don't think it is true, and neither should Dean at this point--and the former was downright silly. Dean is four years older than Sam. He may have, even in their younger days, respected him on some level for his resistance to John, but he definitely wasn't "looking up to him" for most of their lives, and his anger at him for breaking with the family business wasn't all a defence mechanism.
-The fact that Dean seems to have died so quickly after the events of last week. I have no problem with him dying on a random hunt, and don't think it takes anything away from his legacy; they aren't immortal and even their cake runs are extremely dangerous. But I'd really like to have had some sense that they had at least a semi-extended period of enjoyment before that happened.
- The old age makeup.
And the meh:
I find "and they all lived happily ever after in heaven" endings kind of cheap and hollow. Not bad, per se, especially on a show with an established, developed concept of heaven, but given that I have to live in the ordinary world where the best most of us can do is hope that there's something good after we die, it isn't that satisfying to me on a human level to get an ending that rests so heavily on the afterlife. It kind of begs the question of why, in that case, we should care so much about what's going on in Earth. Like, if the vast majority of your existence is taking place in heaven, why does it even matter so much if you get twenty or thirty years more or less on Earth? I suppose Sam couldn't have had a kid if he'd died with Dean, but I'm not seeing anything that would have stopped him from finding a romantic partner (new or old!) in heaven. Or taking up a new hobby, or travelling, or doing any of the many other things that make life meaningful on Earth. I guess there are a number of high-stakes things you can't replicate in heaven: you don't need doctors or scientists, for instance, and since you presumably can't die again, that makes a lot of things less urgent--but again, if death means that you go to heaven, death on Earth is a lot less urgent, too.
In the world of this show, I think it still could have largely worked if the mythology of the show had ever included TFW, in any of its configurations, mounting a sustained rebellion against Memorex Heaven. As it is, what Dean and Sam did over the course of the show still mattered a lot, because the result of all of their various encounters with angels and demons and gods is what enabled Heaven 2.0. But even though that's true, without specific, proximate investment in reforming heaven, it isn't that dramatically satisfying. Logically, I can say "Wow. The Winchesters were key players in all of these apocalyptic events that ended in the replacement of Guck with a better god, who reformed heaven." But emotionally, it just doesn't feel all that weighty, since the issue of heaven sucking hadn't really been one the show had deal with for a while. It was all, understandably, about saving the human world. But then you can't have it both ways, and expect us to be delighted with an ending that is so heaven-directed.
So again: was it terrible? No. I'm basically OK with this as the end for Sam and Dean. But it doesn't feel like an especially organic end to their journeys. A reasonable enough conclusion, sure, but not one where you feel that all the narrative and emotional dots have slotted into place.