1) Tarrlok and Amon are set up as dark foils to Bolin and Mako. Amon and Mako are both exceptional benders with "weaker" younger brothers who they seek to protect from harsh circumstances. The parallels are too plain to ignore.
2) Asami's father seemed to be tied to Charles Lindbergh (famed aviator and noted Nazi sympathizer) a lot this episode. From the outfit to actually flying the planes in battle.
3) While it was really obvious whenever they stopped to show someone pulling their parachute (lest the kids get the impression that they died or anything), this show went to really, really dark places. Horrifically cruel parental abuse aside, we got both a lovingly enacted murder suicide (I could make so Babylon 5 references here, but I don't think Guild would be too appreciative of that) and an obvious near suicide attempt from Korra.
I feel like the show's biggest problem so far has been the pacing. While the original series sometimes went a bit too slowly (especially with episodes like "The Great Divide"), it did give the audience a lot of time to get to know the characters. The Legend of Korra, on the other hand, seems to have gone a bit too fast; quite a few of the plot developments seem rushed and we haven't been able to know the characters as well (Mako, Bolin, and Asami especially). The season really could have benefited from a few more episodes.
While the season finale did seem to clean the loose ends together a bit too well, I was definitely satisfied overall. Despite the nitpicks, I've been loving the show. There is still a lot of ground to cover, and we have (at least) another season ahead. I can't wait to see where it takes us.