I actually covered this across a couple other posts I made. WRT to a single objective in TLA, you're right. TLA was geared toward a younger audience, which is why there wasn't much character development outside of Aang and Zuko, and why the plot was simple and straightforward, i.e., stop the fire lord, end the war. Once that was over, the story in TLA could be wrapped up fairly nicely.

LOK on the other hand was geared toward an older audience, the original TLA fans who were now in their teens. Which means the story needed to reflect the complexity of their lives so they could relate to it, and learn from it. No more simple objectives, or minimal character development. The object of LOK wasn't to defeat an enemy, it was to defeat oneself, i.e., Korra's personal growth and development into not only an adult, but a fully realized avatar.

Were were already familiar with the world and the physics of the avatar universe from TLA, which means it could be the background of LOK, acting as a mechanism to provide the writers whith whatever they needed to pound out Korra's (and the rest of the cast's) characters.

I do agree with you WRT an unfortunate lack of stand alone episodes. It would have been great to see the characters in situations where defeating an enemy wasn't the main goal, but I guess the complexity of the subject matter coupled with the animated medium and short time format required certain sacrifices to be made. Especially when the enemies aren't working with set schedules like a comet arriving, and the sense of urgency was far greater.

I also agree with season 2 being less than satisfactory. Some characters were written very poorly, especially Bolin. However, the avatar Yuan episodes were also very well done, in terms of storytelling and expanding the lore of the universe. They were definitely the highlight of that season for me.