I find the opposite to be true. Fiction, to me, is about subverting the audiences expectations. If you're going to go with the standard model I think you should have a reason for doing it and it should be clear to the reader.
Actually, that is muddled. You should just have a reason for everything in the story.
Well yeah I love subversions and stuff but for example if your world is the standard high fantasy world (which I personally hate but hey a lot of people are better character writers than setting writers) but you call goblins "greedos" or something like that you should probably have an explanation as to why it's that way, especially if they are exactly like goblins in all other respects. You don't have to point out the explanation in the story specifically, but if you really can't think of a reason why you called them greedos in the first place (even "I think greedos sounds cooler" works) then well why couldn't you just call them goblins?
I guess we agree overall though because as you said everything in the story should have a reason to be there (a good one is preferable, however).
Acanthus, about when is your story set? I mean, there are a few (more than a few, but I'm generalizing) sci-fi eras so I'm just wondering which one yours is set in. For example, there's the "just found out there are aliens, woah space is a big place" era, the "members of the galactic council, hey look we have FTL travel now" era, and the "aw yeah b****es we're a galactic superpower, technology can do anything" era.