I hate when I only have half an idea or can come up with an idea. Say I write a killer riff or chorus or verse and then don't have another part to go to. It's so very frustrating. Though nothing is more frustrating than going to play and just not having anything that day. It kills me, I'll get depressed for days over it.
Conversely having ideas, complete songs and such that you can hear in your head but not quite realize is also very frustrating.
Not remembering how to play something cool you wrote is pretty sucky too.