In Hexxat's quest, I agreed to kill the six leaders to get Ki Chin Sang's key.
I killed them.
I talk to Ki Chin Sang and instead of giving me a key or anything, I just get a "- has nothing to say to you" prompt.
(The only thing I can think of that might interfere with this is that I killed one of the leaders via freeze and shattering them. I don't know if this has a different death trigger or whatnot - though other people having this problem seems to indicate that this is just a straight-out quest bug.)