I think my biggest frustration is trying to get through beginnings and set-up and filler in the middle to get to the big parts/scenes that I've been visualizing in my head and looking forward to writing. I am not one of those writers who writes out of order just because they're inspired, and that, along with the fact that I tend to write long stories, means that sometimes I have to wait YEARS to get to those parts. I hate that.
I also hate when I start a story and get far enough along on it to post it and then completely lose my inspiration for it... or get another idea I like better and abandon it for a new story instead of finishing it first.
Also, with fanfic, writing the Boys' families. I usually prefer writing scenes with just one or two characters or, at most, the five Boys together. I don't usually like involving other people, besides my own original characters, but sometimes, for the sake of realism, it's unavoidable.