There are several things that I absolutely HATE in campaign settings:
1. Grim 'n Gritty & Nihilistic Worlds: Some worlds take great delight in informing players that there is no hope. If there is no hope, why bother playing? I want to play in a world where I can be heroic, not get depressed!
2. Cliched Polytheistic Pantheons: Although D&D seems to assume a polytheistic model, I really hate how in every setting every culture seems to follow a pseudo-Greco-Roman pantheon. Give me some diversity! What about monotheism, which has greatly affected our history? What about dualism? What about animism? If there is going to be a polytheistic pantheon, I would at least like to see deities that aren't devoted only to adventuring.
3. Every Culture, Race, Society Speaks "Common": This makes sense for some game purposes, but it gets a little silly when everyone on the face of the planet (including, elves, dwarves, etc.) speaks the same language without any variation in accent or dialect.
4. Magic is Common and Mundane: Although I really don't like grim and gritty games, I also don't like games on the other end of the spectrum where magic is such a common thing that even the lowliest peasant has some access to it. At such a point I think that it gets silly. Magic ends up emulating technology (Crystal Ball telegraphs, Continual Light street lamps, etc.) Also suspension of disbelief seems to break down. (Why worry about getting wounded or killed if any local cleric can fix you up?)
5. Worlds with no Internal Consistency: I like high fantasy, and don't care for too much realism, but I hate worlds where things seem to be thrown together without rhyme or reason.
6. Carbon Copies of Real World Cultures: Let's have some creativity, rather than taking real world people and places and changing the names slightly. (7th Sea's Theah and Kara-Tur are the worst offenders.)