Articulation.
Give me a day or two, I can craft the beginnings of a good character. Give me a strong first inspiration, and I'll build you a world. (Not immediately of course, but it will get done eventually.) Give me a plain, boring plot and I can twist it into a good one. Tell me that I have the worst grammar in the world, and I will sit proofreading my chapter until it's error-free. You can even give me writer's block and I'll find a way to mentally kick myself and begin writing again. (I actually did that once, after half a year of pause.) But the one thing I have the most trouble with is description and the process of getting my thoughts down into words. I'm not sure why, either. Maybe my vocabulary isn't yet vast enough, or I haven't read enough stories to see other methods in action.
In most cases, I can see scenes pretty clearly. But every time I sit down to write them, I draw up a blank. Of course, I can force myself to describe things to at least fill up some pages, but the result if me repeating words and phrases. (Have you ever read books when an author uses the same words and phrases over and over again? That's me in my first and second drafts.) For the life of me, I can't find a way to express the movement of fabric other than to write 'it billows in the wind', or to describe an epic character appearance in more ways than 'it sprang from a bright flash of light'. Once, it actually took me two days to see that writing 'it flaps in the wind' was another and better way of saying 'it billows'! Also, what sometimes happens with me is that when I find a phrase that I like, I start using it over and over throughout the story. I swear, in some of my stories, my characters have 'nodded' and had their 'lips slowly spread into a smile' about a thousand times.
My first drafts are, literally, long blocks of text disguised by paragraphs and spacing. No body, no structure. (Either that, or they're bare and feel rushed when you read them.) The characters and story could be as lovely as they wanted to be; the structure would still be boxy and boring. A while ago, I thought that the soluition to this problem was to describe everything, but that didn't work out either. I started reading a bit more, and revising a lot more. That's the only thing that helps, and it can take days. During the revision process, I have to either trim them or beef the scenes up to make them perfect. The more time I spend on it, the better the scene gets.
I also have an issue with under-writing a dramatic scene and skipping over really emotional moments. This ties in with my mentioned problem, and I have to edit to get it right.
Yes, this is annoying, but I've learned to cope with it. (Partially.) It means I have to spend an extra hour (sometimes day) editing my chapter and making sure I get it right. It's worth it, though, since I love to see a scene of mine come alive.