My biggest shortcoming as a writer is the fact that my brain hemispheres don't know how to work together. I'm a right brain dominant person, so words aren't always that easy for me to put together. When I write, though, my dominant hemisphere has to surrender the reins to the left side, but once the logical side takes over, it doesn't even want to let the imaginative side contribute. It's so concerned with sounding good and being grammatically correct that it doesn't even allow for any actual story to take place.
Somebody once told me that a good way to judge your own writing ability is to summarize one of your chapters or scenes (or several if you have to, of course). If you spent the entire chapter detailing things, but not actually moving the story along, then you haven't really done much for your readers; or even yourself as a writer. So I looked back at some of the things I've written, things here and things elsewhere, and I've noticed that I was that writer. The one who focused so hard on detail that I missed the opportunity to actually tell a story, let alone say anything.
I believe that this is a direct result of the left brain being selfish. Normally, I can easily picture a scene and how it would go down from beginning to end, but once I start writing, once I switch on the wordy side of the brain, it seems to disrupt or block out my creativity. It's crazy, and it really bugs me.