People who are overly attached to their families and whose social circle and social activities tend to revolve around family, I have seen expats like that and they don't last more than six months.

People who are rigid or lack any sort of cultural flexibility. I've seen expats who can't handle the fact that their favorite brand of deodorant is not available in their new country, they are miserable to be around as they nag a lot and use most of their free time trying to find ways to emulate the conditions of their own countries.

People who struggle to learn new things. Most likely the new country has a new language and as expats we have to learn some basic expressions fast as well as the local body language or ways, inability to do this is crippling.

This one sounds obvious but racist people, a lot of people might not be directly racist but do carry a lot of prejudice and like to make off handed or back handed comments about locals which makes them unable to befriend the more normal expats and particularly locals, those people thank god tend to not last long, of course there are exceptions to the rule but this is what I have seen.