I like the interactivity of it. I write the plot outline, but the players write the story.
I have a couple of adventures that I have designed to be playable in different systems and genres (with tweeks, of course) and each time, the outcome has differed- sometimes significantly. One group thought one of my modular adventures was the best adventure they'd ever been in (Darksun), another group thought it was "Hella-cool" (Champions), and in another group, it was enough of a dissapointment that it ended the campaign (RIFTS). Its gotten mixed reviews at gaming conventions (mostly positive, though).
It wasn't the adventure itself- it was how the players interacted with it. You know- "It's not the destination, its the journey."
What do I hate? Problem players, but I can't decide which I hate most- those who are continual headaches for me/the others/everyone, or those that leave and don't tell me why, or how I could improve things.