Maybe "Appa's Lost Days" could be seen as filler, but it really made you feel sorry for Appa. I couldn't help but thinking that save for some off-kilter timing, he and the gAang would have been re-united a whole lot sooner. And oddly enough, the only reason Appa is in Ba Sing Se is becuase that's where Aang when to find Appa. And it did set-up a confrontation between the gAang and the Dai Li; I'm thinking the season finale will have Aang causing a lot of destruction in the Avatar state, possibly becuase of Aang finding that Appa has been severly mistreated.
I imagine that lots of fans went "squeee!" at the sight of chibi-Aang and chibi-Appa (I'm a male adult and I still thought that scene was cute).
The guru guy was kinda neat, especially how he just waited for Appa to fall asleep before getting up.
I didn't really care for Ty Lee's "prettier than you" remark during the fight, but since she's been shown to be a bit of a random bubblehead, I guess it sorta fits. I was rather impressed that Suki was able to hold of Azula as long as she was able (I don't think she got killed, but I doubt she necessarily won either).