Reverse clickies
Sharp edges and threads(not to be confused with aggressive knurling) and poor ergonomics in general
visible PWM
Useless modes that are not tucked away where I will never see them
UIs that require a flowchart and an explanation to understand
Tasteless laser engraving
Poor finishing before anodizing
Poor anodizing
Purple, pink, and green tinted "white" LEDs
Threads that feel like a pepper mill
Bad/no O-rings
Yeah, I think that's enough.
EDIT:
Hmm... this seems to have become a pet peeve list, not a deal breaker list. I've moved the pet peeves down here:
Fingerprints inside sealed bezels
Uselessly tiny lanyard holes
Useless clips (too weak, too damn big, too damn small, and just plain BAD/fugly - I would rather have no clip at all and just add a McGizmo clip)
Lights that have poor proportions (SF E2e=fantastic, Fenix TK30=too long, or not wide enough - take your pick)
GITD on a tactical light
Polished/SS on a tactical light