A few obvious examples:
Aggression: too much, and you go to jail. Historically, prison conditions were awful enough in the West that this likely exerted an upper bound on the population’s criminality by eliminating violent people from the gene pool.
But too little, and you get taken advantage of. You can’t compete in job interviews, get promoted, make friends, or ask people out on dates. Aggressive people take your stuff, and you can’t protect against them.
From getting jobs to getting mates to not being mugged, a little bit of aggression is clearly a good thing.
Intelligence: High IQ is tremendously mal-adaptive in modern society. (This may always have been true.) The upper end of the IQ curve basically does not have children. (On both micro and macro levels.) I’m not prepared to say whether this is a bug or a feature.
But, low IQ appears to also maladaptive. This was certainly true historically in the West, where extremely high death rates and intense resource competition left the dumber members of society with few surviving offspring. Dumb people just have trouble accomplishing the things necessary for raising lots of children.
Somewhat above average IQ appears to be the most adaptive, at least in the present and possibly historically.
Height: Really tall men have health problems and die young. Really short men are considered undateable and so don’t have children. So the pressure is to be tall, but not too tall.
(Speculatively) Depression: Too much depression, and you commit suicide. Not enough, and you’re a happy-go-lucky person who drops out of school and punches people. Just enough, and you work hard, stay true to your spouse, don’t get into fights, and manage to do all of the boring stuff required by Western society. (Note: this could have changed in the past hundred years.)
Sickle Cell Anemia: I don’t think I need to explain this one.
(Also speculative) Tay Sach’s: Tay Sach’s is a horrible neurological disease that shows up in populations with evidence of very high recent pressure to increase IQ, such as Ashkenazim (one of the worlds’ highest IQ groups) and Quebecois. There is therefore speculation that in its heterozygous form, Tay Sach’s may enhance neural development, instead of killing you hideously.