Well, I didn't know I wrote mystery until I turned in my third manuscript to my editor. I'd screwed up big time and my editor had to inform me that I wrote mystery. Had I known beforehand! :)
But fun factoids? I think the best part is setting up the clues along the way along with the red herrings. It's hard to find that line of how much is too much? Will this clue give it away? Have I gone too far with this scene? It's all choices and it's fun.
Also, with recurring characters, you really get to know and love the people you've populated your world with. They become like family and you think about them ALL THE TIME even when you aren't writing.
And figuring out new and unique ways to murder people is ALWAYS fun! :)