Having a brain that feels like mush whenever it is writing time.
I have a toddler, so my days are spent having Frozen stickers stuck on my face and stopping said child from dropping her ice lolly into a flowerpot and then continuing to eat it. This is obviously great fun, except by the time I get round to writing (aka about 8PM), my brain feels like it's been scrambled and I can't even remember what my story's about. It is RIDICULOUSLY frustrating because I spend all day looking forward to that time when I can write and when that time arrives, my brain lets me down.
Pre-parenthood, the most frustrating thing had to be that first draft, simply because of its sheer AWFULNESS. Some of that content I wouldn't even shred for the cat's litter box, it's such poor quality. Editing, however, I love. Loved it then, still love it now.