I enjoy creating an atmosphere where people can forget about reality and enjoy playing the role of a, well, "hero biscuit".
The NPC character creation process is what I like the best... and I'm not talking stat blocks, either. I enjoy creating "real" NPCs on the fly; attitudes, personalities, quirks, jobs, relationships... and how they fall into the PCs lives. By shaping the NPCs around the PCs, I give the PCs a sense of worth, of being someone important in the game world.
What I really, really dislike is the whole tactical aspect of D&D 3.x. I had the pleasure of having a 'wargamer' style player join my gaming group; he owns no less than 1000 bucks worth of miniatures, plus a pretty cool dry-erase tile set that comes in 12 pieces and has a grid printed on each tile (plus, they're modular, so you can up and move 'em to make an interactive dungeon map on the fly).
But... he knows the tactical rules inside and out and treats the game more like a wargame than an RPG. Couple that with the facts that I'm used to winging most fights and that I've generally used a grid only for large numbers of opponents, and I get pretty frustrated. Now I have to worry about things like moster positioning, whether they're being flanked, what actions are best for the monsters to take while accounting for their intelligence (or lack thereof)... and I'm not a strong tactical thinker, so I end up making silly mistakes that the players can capitolize upon. :\ But, it's a learning process...