Another great one you got there! IMO this would be perfect for a large pharma company to brand their depresssion meds. Outside of a large pharma company owning this i dont see too many uses for it, if i were you i would definitley put up a minisite with depression info on there and maybe some affiliate links. I guess a psychiatrist could use this domain for their practice. Maybe some sort of a world relief charity organization could definitley use this domain - it would be extremely effective in a marketing campaign.
The only thing that discourages me is that there are not alot of ads for the terms "sadness" which may limit your ptoential pool of end users. But if I were selling I would not take less than $2.5k for this domain.
Thanks for the nice appraisal. Yes, I must agree that this domain would probably be best used in the medical world. I am seriously contemplating putting up a nice minisite with affiliate links.