They shouldn't have had a writing problem with the 12 episodes we got. Let's take British short series for example. Yes, they're an hour long, but usually there are 3 - 6 episodes on average. 6 one hour episodes = 12 half hour episodes. 

Use the standard story telling arc: introductions & trigger (eps 1 - 3), quest & surprise (eps 4 - 6), PROBLEM & CLIMAX (eps 7 - 9), and reversal & resolution (eps 10 - 12).

While they did try to cover that with the 12 episodes they mixed in too much unstable character actions and little development. It was rushed and poorly written because there wasn't any real character growth it was a "oh we better have [this character] do [this] or act like [this] because we need [this] to happen even though that made no sense for them to do [that]."

Mako being an example was very wishy washy. Some will argue people do do that in real life. Yes... but it was very much established early on that he didn't really want much to do with Korra. And then what was up with Tenzin and Lin? They didn't even react slightly concerned for Korra's disappearance yet Mako was doing a lot of the "over reacting". More like the only one really reacting to anything because Makorra was going to be 'endgame'. 

Lightning bending, blood bending, and the plastic surgery are another little things to just quickly answer questions without real explainations. Lightning bending is supposed to be a very high level firebending ability. Even in 70 years not everyone and their brother and sister can have a balance between their yin and yang. The only point of LOK was that there was an huge IMBALANCE between yin and yang with the humans and spirits.

Also unless someone else discovered bloodbending. Katara would have never told anyone about bloodbending and it would have died with her and Hama. The only thing I can think of that would explain how people would know about it is with the Fire Nation people who Hama took prisoner. They were bloodbended on and might have told their friends and family who told others. And some curious waterbender might have later tried it and succeeded. But I really doubt this would have happened as easily as Bryke made it sound.

Plastic Surgery... ugh... yes, it's been around since B.C.E, and was happening a little bit in the early 1900s, but it was never that advance even in the 1930s standard which LOK is based around. It was again a quick answer to the question for Amon and Tarrlok.