I'm really surprised at the number of people who are saying that they flat-out won't play in an otherwise tolerable game if the DM dares to make them roll dice for stats and HP. It's almost like the DM expects them to play some kind of largely luck-based dice rolling game!

My biggest dealbreaker is obvious inflexibility in a DM, and I don't mean in a railroading sense. In fact, I think the DM's story is pretty important, and if the DM has an overarching plot then I'm fine with letting them set things up as they please (so long as they aren't restricting the characters.) But no DM has every facet of their world in place and every race's cultural history set in stone. So long as it doesn't interfere with the DM's story, I would hate to hear "No, your Elf Barbarian can't be the sole survivor in a great war because I don't have any Elf wars planned in my background's history." (Again, if the pacifism of the Elves is a key point in the DM's story that's valid, but if this is a werewolf-hunting campaign then it shouldn't come up at all.) Similarly, I see absolutely no reason that a Faerun-inspired setting should be disallowed Changelings or Warforged or Shifters, or an Eberron setting should lack Sun Elves and Deep Imaskari. I would be wary of playing in a campaign where generalizations like setting are so absolute that it takes away from the enjoyment of the players.