1) Pott's art style is very much the kind you would have seen in old children's books. It has a very nostalgic feel to it, and while the cartoon proper has changed it to make it easier to animate (or, in FUCKING MOYNIHAN's case, completely ignores attempting to stay on-model), it still retains the same pathos behind it. If you combine it with a timeless setting or, in this case, what appears to be the late-90s, you have a group of people who will look at it with fond emotions on both a personal and collective level.
2) It has writers who care about what they're writing, or at least appear to. It's got stupid-funny word puns, it's got smart dialogue, but the most important thing, to me, is that it doesn't pull any punches when it comes to emotional impact, especially when it involves Hedgehog and her family. It's a show that does a great job of sucking you in to a level of comfort, punching you in the gut, and then making you feel like you deserved it. In this way, it's effective at making you remember all of the bad things; however, the comfortable parts of the show are there to soothe the ache.
3) The child voice actors are great. Everyone remembers the old Charlie Brown cartoons, despite their kindergarten VAs being awful. Having actual pre-teens voice the characters, save the witches and adults, makes the kids feel believable, and the fact that the kids are good at their jobs makes you want to see more of any of them, rather than praying for a Susie scene so she can talk instead of them.