As a rule, I only write stuff if I feel like I have an opinion or take on something that isn't very common. I've abandoned half-written posts because I realized that I am not saying anything particularly unique or interesting, and I don't really see the point of writing about it.
Granted, being uninteresting has never really stopped me from writing long comments on HN...
(This takes a lot of pressure off - I publish notes, half-finished thoughts, stuff I'll "get around to later" with no shame).