E. E. Knight has posted a list of 20 genre writing sins to his LiveJournal; it's a pretty good list. I'm teaching the Viable Paradise science fiction/fantasy workshop on Martha's Vineyard this year, and many of these items have already come up in the lectures and critiquing sessions.
The Joker: Smiling, and its evil cousin, the grin. I've read entire chapters where characters do nothing but smile and grin at each other, as though they're living in the Treehouse of Horror Simpson's vignette where Bart has mental powers like the kid in the Twilight Zone episode "It's a Good Life" and everyone has to keep on a happy face…
# Don't open the airlock! Another thing that bugs me is a scene that seems to take place in a vacuum. No sense of time, place, no indication that anyone has a history or is concerned with anything other than what's on the protagonist's mind that very second. Please, establish a time and place, even if it's just "midnight at the oasis," before or as you start the action! And remember, everyone in the story has problems of their own.