Korra was originally intended to be only 7 episodes and nothing more than that, I believe. That's mainly why the character development was lacking and why it feels rushed at times. But that being said, while Korra has its flaws, I found it to be an acceptable sequel to TLA, and I'm interested to see how the characters develop now that it's confirmed they'll be having more seasons to work with.
The advanced bending thing can be explained pretty easily. In the Promise and the Search (the two Avatar: TLA comics that explain what happened in between TLA and Korra), you see that characters like Toph starting up her metalbending school, which will most likely expand as time goes on and more and more Earthbenders are able to master metalbending. I'd imagine that the Fire Nation restricted the lightning technique to the masters in the royal family or military higher-ups (and with masters like Jeong-Jeong on the run, no one was able to teach the lightning art to anyone other than the royal family), but after Zuko takes over, that information is free and open for everyone to know. Zuko, Iroh, and Aang most likely teach and pass on the lightning and lightning redirect skills to other firebenders, and naturally, it becomes commonplace. I'd equate it with the rapid advancement of technology, in a way.