Powerpuff Girls managed to be bad enough to have several bad reboots until I stopped caring or paying attention to them.
Transformers started as a glorified toy commercial and even at its best didn't wander far from that, but the Michael Bay era almost killed my fondness for the setting and its characters. Almost. The Bumblebee movie seemed like a sincere apology attempt.
The bloated and unnecessary Hobbit film trilogy. :why-angel: Check out the short and beautiful Rankin Bass animated movie instead.
Most Disney live-action remakes of animated films sucked, but the Mulan one was especially insulting and utterly missed the point of both the literary original and the animated film that was a lot more respectful to it.
Why is it a weird criticism? Fantasy stories' sequels often suffer from this same problem. Writers failing to understand that mysteries are good and that allowing people to speculate and infer is okay. The biggest failing of Legend of Korra as a fantasy series is that very problem.
Of course it fails in many other ways as well, but I had to start somewhere, and that's the part about it I dislike most.
Other than explaining where the avatar spirit came from, it didn't explain much else in my opinion. It sorta retconned how bending came to be in humans and expanded on the spirit turtle a bit, but I don't think we knew more answers to questions aside from those. It raises some weird questions of its own, like why is lava bending rare, why is platinum immune to metal bending, how could tarlok and amon bloodbend in the opposite lunar cycle to what was established, why is the world westernizing when there is no west to adapt to?