I'm still entertained by this. It's easier for me to write about my Dislikes than my Likes, which is something I'm pondering.
LIKE: HOMEMADE SETTINGS
I enjoy building a world - that's a huge part of the fun for me as a GM. As a player, too. I like coming up with little social, cultural, and political details. It's fun when they have some implied connections to each other. You can use them to suggest possibilities for future adventures. Several times in Gorbadin, players discovered small stone carvings of odd elongated heads - who created the urgol-faces, and why? We never found out, but we could have...
That kind of freedom comes when you create the setting. Things can go in whatever direction you want. They'll reflect the creators. My Gorbadin campaign was easily the best one I've been involved with, not because I created it, but because everybody created it. We all added stuff that we thought was cool, and you know what? That made it cool.
And I have yet to discover a pre-written setting that was satisfying. I feel constrained by the original designers' intent, which is often different than my own. Deadlands is a neat idea, but I can't get past their handwave about slavery. My fondness for 7th Sea is well-known, but the established history is sketchy and has a tone I don't enjoy. Even my perennial favorite Castle Falkenstein needs some revamping before it's something I want to run. I just like stuff better when it's my stuff, or my group's stuff.
DISLIKE: DUNGEON CRAWLS
This shouldn't come as a surprise. I don't like dealing with the physical environment, I don't like maps, I don't like solving puzzles. I had a chance to write for the fairly-successful Dungeon Crawl Classics line, and I blew it - it's the only time I've punted something, but I just could not get myself to write dungeon crawls. They're sterile and boring. They waste all the social and interactive potential of the RPG form. They're arbitrary. And they have terrible stories, when they have stories at all. Dungeons hem you in and take away your choices; RPGs should expand your options and give you freedom.
Judge for yourself. Here's a D&D 3.5ish rewrite of the Gygax classic "Tomb of Horrors," as done by Jason Alexander. There are some clever traps here, all right. Yep. Clever. Trappish. Gonna kill you right dead, they are. Whee. Remind me why my character is doing this again?
Overall, I liked D&D 3.0 and 3.5. But when I ran across the DMG sidebar about "Why we're returning to the dungeon," I swore out loud at my book.