To put in too much dialogue because I love writing it especially when I've got my characters well set up. I do enjoy writing narrative, especially when it pushes the story along at a cracking rate, but my soft spot has always been for dialogue given it can be used to show character traits well (not just by what is said but in the way in which it is).
To keep writing a story when I've gone past the natural ending for it because I'm having far good a time with this tale and don't really want to end it! Be ruthless. I do have to kill my darlings!