It's not just that the ending is perfect (though it absolutely is). It's that at every point, in every season, when they could have chosen the easy, dumbed-down route for continuing the show, they... didn't. They didn't do that, and it's hard to express what a difference that makes.
For example (without spoilers), the way season 1 ended set up, for a lesser show, an easy rehash for season 2, where the same characters get up to slightly different versions of the same season 1 hijinks.
Instead, the show runners packed that painfully obvious concept into the first half of the first episode of season 2, then moved right past it. They could have milked the concept for a whole season, but they didn't want to. So instead they acknowledged it in the most hilarious possible way, then got on with the actual story.
Goddamn, now I want to watch it again. It's just so good.
Thanks for the insight, it's one of the few shows I've rewatched.
There's an official podcast with the showrunners and the cast members where they do post episode discussions. It's a great listen after watching an episode.