Well, IMHO...
Steampunk Fantasy: I've just never cared for this sort of setting. I've always envisioned there being a balance between magic & technology: if one increases, the other must/does decrease somehow. Now, oddly enough, I don't mind stuff like d20 Modern, or superhero games with magic (which I always view in a more comic book-ish style as it is), where magic & above-average tech mesh.
D&D in Space: I didn't care for Spelljammer, & Dragonstar didn't grab my interest at all. Not interested in any sort of fantasy/sci-fi mix along these lines.
Planescape: I didn't care for the idea of having adventures in the Outer Planes--to me, it removed the awe factor of these planes--I mean, this is supposed to be taking place in a D&D world's equivalent to Heaven & Hell (plus a few other places).
One-Terrain Worlds: I like campaign settings to have a variety of terrain (& thus allow for a variety of adventures)--thus, I didn't enjoy the desert-dominated Dark Sun, nor could I really get into an all-aquatic campaign (sorry, Aeolius). I mean, I enjoyed reading Wizard of Earthsea, but I doubt I'd enjoy playing a campaign set in that world for very long.
Gloom & Doom Worlds: Both Ravenloft, as well as WW's World of Darkness fall into this category. I think that horror themes & elements can be incorporated into any game, but & don't like playing games that are heavy into horror. I think that a good game can have & allow a diverse array of themes/genres--1 adventure may be horror-inspired, while others may include an undersea adventure, a dungeon crawl, a city adventure, a wilderness survival test, a comedic romp, a game of intrigue, etc. Basically, a nice & healthy mix of stuff instead of games focused on 1 or 2 themes.