Not sure what you were expecting from Sokka and Katara. Sokka was always the idea-man, not really much of a swordfighter. He has that great scene on the airship where he saves Tof. And Katara? Heck, she performed a major feat.The fight between Aang and Firelord Ozai was the best part, and took up like half an hour or so by itself, though they split it up a bit. It seemed really appropriate when Aang made a sort of planet-like surrounding of all four elements around him, and kicked arse. Toph's fight on the airship was sweet too.
I think only two minor bits disappointed me; Sokka didn't do much swordfighting, and Katara didn't do a lot with her waterbending skills. They were both a bit more impressive on the Day of Black Sun or other times. Otherwise, an awesome finale.
It did look like they're planning some kind of sequel though to cover a few loose ends, like whatever happened to Zuko's mother. They didn't show what became of Azula, either. And I'm curious if Aang would ever be able to re-establish the Air Nomads, or if he really will be the last airbender in the history of that world.
As to loose ends, I'm not sure we need anything spelled out for us. Looks like everything starts coming up roses.