Where to begin? So many to choose from. Everything from story development to characterization to authenticity can be challenging to writers, and that's just skimming the top of the barrel. I'd have to say that, at the moment, my greatest challenge is within the realm of style.
It's not that I don't know what my style of writing is or that it doesn't come naturally. It's more that I let myself follow my mind's eye and ear and my heart, to the exclusion of paying attention to the proper writing conventions. For me, that translates into blocks of dialogue that are not broken up by action or internal monologue or environmental factors. And blocks of internal monologue, and blocks of exposition. When something is fascinating to me, I get tunnel vision.
This is not always easily fixable on rewrite, but it is fixable. The more I work at it, the easier it becomes. I have to admit, however, that at this point, I'm not catching all the times I make this mistake, not without help. A novel is a big, sprawling octopus of a narrative, and it's just plain hard to pin those powerful, squirmy, suction-cuppy arms down where they belong.
Perhaps the most important thing I'm learning from my challenges is how incredibly valuable a good critique group is. It's so invigorating to recognize problem areas and actually fix them! That's what happens with good critiquing and determination. With the help of my wonderful critique group, I'm thinking that my tunnel vision will someday be a thing of the past.