Plot: The cool thing about Coven is it started with a clear intention and laser focus. We knew the biggest villains: Fiona is a villain to her own coven, Marie Laveau is Fiona's biggest enemy, and the witch hunters are the greatest threat of all. The search for the new Supreme turns the other spell-casters against each other. Coven ranks so high because it's such an iconic season. It gives some of the most quotable lines of the entire series ("Balenciaga!" and "Surprise, b*tch" and "Wear something . . . black" are a few of the common faves) and it delivers amazing fashion and stellar performances. There's a campiness and a levity that the first two seasons lack. That's where it's brilliant.
Ending: What I hated about Coven was its inability to stick to its own mythology. At the start of the season, each witch is supposed to have her own special power, and that's it. By the end, they all suddenly had plenty of abilities, and only the true Supreme can perform all Seven Wonders. The whole season turns out to be a mess. I will say it's gratifying to see Cordelia's ascension as the next Supreme and the sweet justice of Fiona landing in her "Knotty Pine" purgatory with the Axeman. But it's not enough to make up for everything else.
Scariness: As a disadvantage, Coven isn't that scary at all. That said, there's some scary sh*t when it comes to Papa Legba and the different versions of Hell. Everything involving Marie Laveau and the Minotaur and such is pretty freaky as well. On the whole, it's clear that Coven isn't trying to be the scariest season, especially after Asylum. And that's OK.