By "mood" I mean both the general assumptions about human nature, and what the setting does to them.
Star Trek - at least Next Generation, which I am most familiar with - is pretty optimistic: Most human are basically decent, if you let them, and will do the decent thing if given a chance; aliens, too, are doing that which is the decent thing from their ethical base. Most the main characters do what they´re doing for idealistic reasons - because it´s the right thing, because they´re continuing their father/mother/whatever´s legacy, because they´re curious and want to see the galaxy, but generally for a lust of money or power, for revenge, or some other "bad" motive.
Warhammer 40.000, on the other hand, is grim and cynical to a ridiculous degree - basically the lucky people are those who are killed and then
eaten by a demon. There are NO
good guys - at all. Paranoia goes in the same direction, except much more for the laughs than WH40k.
Call of Cthulhu can have idealistic characters, but they tend to get eaten and/or go nuts; but, so do cynical characters. There´s really no long-term victory, and just staving off defeat for a while and surviving in the process usually counts as an unqualified success. So, yes, grim dark mood, except no laughs here.
So, which one do you prefer? Optimistic and idealistic, or dark and cynical? Or maybe something in between?
Personally, I find myself drawn more and more towards the darker end of the spectrum - though not to ridiculous extremes like WH40k. It probably has to do with me becoming more and more cynical overall where mankind as a whole (which tends to feature quite heavily in SF settings) is concerned. Don´t get me wrong - I´ve met plenty of wonderful individuals, but large groups, or humanity as a whole? Come on...
Anyway, I don´t really go for black-and-white settings any more.