The pacing is off, they try to fit as much personality and character in one season where the original took 3. Too many new characters and new dynamics to rely on the established setting and should have been padded out better. Seriously, what took half a season to hint at the real crises at hand in Last Airbender, was planted in the first 3 episodes and promptly resolves itself at the end of that season, which was only 13 episodes long.
But the bar was set pretty high to begin with.
That said, Korra is still top notch stuff. Great animation and action, the world is still amazing and the evolution of it works so well. The characters are still Endearing and charming with very human qualities and flaws. Korra as a character is fantastic.
Aside from the pacing issues, it also seemed to get some flak over how a particular romantic arc was handled, but I found it to be a very mature subject matter with a believable outcomes that was more sour grapes then happily ever after.
Again, the major gripe with the series was pacing. But I think it excelled at other things too. It deals with more mature subject matters then the original series and humour of the series is based far more heavily around character interplay then general goofyness or Sokka-isms. That isn't to say there isn't goofyness or Sokka-isms, but there is a lot more going on with the humour then throwaway gags which were used heavily in the The Last Airbender.
I suppose it's shortness is both its strength and weakness. On the one hand, it's far more focused and there is less fillery material to pad out each episode. That means most of what happens serves an over all purpose. Unfortunately, that also means there is less time to really forget about the crises at hand and just enjoy the characters as they find ways to add levity to their burdens. TLA was great for that.
It can never take the spotlight from TLA, but Korra can be an enjoyable watch on its own as an expansion of the TLA universe, rather then an attempt to ape its success.
