Whooo... so much to cover. But first: 6/10
Yeah, I know it's low, and I probably would have put it higher, except that this was the finale to a spectacular show and I was honestly just expecting it to be better.
I'll write my issues with (mostly) the scripting as they come to me, but before I do that I'd like to offer my take on something that seems to be bugging a bunch of people - the lion-turtle. I did not see this as a deus ex machina device (despite the quite literal intervention of a god :-P ), but instead as an elegant solution to the problem of the impossibility of fighting for peace (it's a contradiction...), as well as a clever way to work within Nick's content requirements. Additionally, the need for an alternative to killing Ozai was a theme throughout the episode, and Aang's position against killing in any form was established way back in season 1. On top of all that, the concept of the avatar lends itself to fantastical powers that seem to come out of nowhere; at least Aang was personally responsible for defeating Ozai and stripping him of his powers, making this nothing like the deus ex machina at the end of Siege of the North. THAT one was blatant.
I thought Katara freezing herself and Azula was pretty cool, too.
Now for the problems...
1. A lot of people seem to be calling the fight sequences "epic", but for a series that dedicated so much time and effort to its fight choreography up to this point (they hired a martial arts master to consult for the duration of the show), I found the panoramic shots of huge blasts of fire (which characterized all of the fights) to be extremely disappointing.
2. The entire thing seemed way too rushed. I don't know if this was a thought-out scripting decision or a marketing decision (i.e. keep the entirety of the finale to under four episodes so we can show them all at once as a TV-movie), but it really bothered me, especially after the show burned the previous two episodes on a side story that went nowhere (Katara get's to find out what happened to her mother! She... died, ambiguously as always. Wait, we knew that. Ok, so se died to protect Katara, which I suppose we didn't really know, but I'm not sure anyone was expecting anything different, and the whole episode could probably have been summed up in a single flashback sequence.) and a meta-episode (which was funny, but not worth the trade-off in space to develop the conclusion more fully). The pacing for the entire third season bothered me, and this was by far the worst.
3. Azula's breakdown was so close to being great that it was all the more tragic when they dropped the ball and turned her into a dishonest caricature of someone suffering a complete emotional breakdown. Again, this probably could have been done better had we seen her paranoia develop over a couple more episodes. Also, her fight with Zuko brought up a bit of a plot hole - one isn't supposed to be able to bend lightening when one is in emotional turmoil, but there Azula was, blasting away.
4. The wonderful scripting of the romantic relationships between the characters up to this point was pretty much abandoned - once again it seemed there wasn't enough time to both finish the plot and explore the characters' relationships, so Mei and Zuko get 15 seconds and Aang and Katara get one scene in The Ember Island Players where she flips out and then everything is (not) resolved with a cliche kiss at the end.
5. After all the buildup surrounding the Order of the White Lotus, it turns out to be five old men (apparantly we've met the entire order already) who manage to retake Ba Sing Se in two minutes of overpowered, poorly choreographed fighting. Eh...
6. Major plot hole: Aang unlocks his last chakra with a blow to the back? I thought he had to give up worldly attachment. Of course it was explained that the avatar CAN'T give up worldly attachment as he is permanently bound to the world so...? Something tells me the writing staff needed a few more brainstorming sessions.
So that's all I can think of for now. I may be back to amend or redact certain statements after I've watched Sozin's Comet again.