THE FUN:
*Writing a first draft. You can only do it once per story, obviously, and for me it's the most fun. It's the first time I get to see the characters actually doing and saying the things I outlined. Very gratifying.
*"A-ha! moment" #1. When a new idea for a story comes to mind and I piece together a quickie synopsis, premise, plot in my head. Can't wait to jot it down.
*"A-ha! moment" #2. When I finally figure out how to resolve a dilemma in my plot that's been bugging me for weeks/months.
*Reading the story for the first time in months after revisions. I run across new chap's I forgot that I added and most of the story seems almost as fresh as the first time I revised it.
THE NOT-SO-FUN:
*Revisions. The first revision is actually not so bad, because the story still seems fresh. But when you get into the tenth or twelfth revision, or you have to revise a single chapter for the 20th time, it's brutal. Realizing I have to cut a hundred pages: not so fun.
*Beginnings. I never get them right the first time. It's usually the first thing I revise and the last thing I still need to revise yet again after I've revised everything else to my liking.
*Fatal Flaws. When I realize something in the plot doesn't make sense and I have to fix it, and either I'm not sure how, or I realize I have to change damn near the entire story now. Ice pick in the ear type of moment.
*Querying. The worst part of the process for me. Hate researching agents, sweating over the query itself, then the dreaded waiting game after I've mailed them out.