I used to write fast. Not blazing fast, but 2000 words in a few hours wasn't out of the ordinary. 2000 words for me now... if I hit that in a day, I'm over the moon. A part of this slowness stems from the fact that I've switched how I write, going from pantsing it to editing as I go. Heavily. But there are a couple of things in particular that kill my momentum when I'm writing/editing:
1.) Boring sentences. Most books have them.
I open the door. Sure, you can make that
I open the door, wondering what horror next awaits or some such, but sometimes a door is just a door and your person needs to go outside without having thought tension involved. However, I don't want my person to just open the door. I want to expedite the boring to the point that it's either non-extinct or folded in well enough to disappear. Pain in the ass.
2.) Segues. Moving from one scene to the next in a fluid, logical manner, while keeping it interesting, informative. Relates to number 1 b/c lots of time my segues involve a change of scenery... damn doors.
3.) Dialogue. Used to come easily. Now I am very particular about trying to keep it concise, in voice, and informative. The biggest problem with dialogue for many, I think, is that it comes too easily and thus can allow for scene looseness, either in establishing an authentic environmental atmosphere or propelling the story in the correct direction. Figuring out how to convey the scene feeling in a believable, entertaining, and propulsive way is one of the most challenging elements in writing, IMO.
What about you? Which elements give you the most pause?