The single most boring and frustrating moment is when I sit down to start drawing thumbnails of an upcoming chapter and. nothing. happens. I just sit there, staring at a blank page, for HOURS. I'll try drawing a layout for a page 15 times, and every time it comes out wrong.
Once I've managed to get a couple of page-thumnails right, it's like that big invisible wall crumbles and everything gets easier - but the first couple of pages of a chapter? Disaster every time. ;_;
The most FUN part of making comics is probably, uh, everything else that isn't "draw repetitive details in environments, including but not limited to rooftiles, bamboo forests, paving stones and stairs".
I really genuinely do enjoy 99% of comics-creation.