honesty.
there, simple ^^
if i don't like someone i'll let them know. if they act stupid, i'll let them know. if i respect sth they did, i'll let them know. if an idea is shit, i'll say so. if someone asks for my opinion or advice, i'll give them both - honest
honesty doesn't necessarily mean to be an ass (althoug i like that very very much ^^ ), but i will be rude when it appropriate. usually i'll be neutral about stuff; for example, when someone talks dumb diarrhoea, i'll tell that they're wrong and why, in a rather neutral way (neither rude nor polite. mostly ^^ )
fun thing is, stupid people tend to take it personal when you correct them; although it's not about them but about what they said/did.
which is one of the reasons why i avoid discussions with certain people, unless they ask for my opinion.
i honestly (ha, see what i did there? xD ) do not understand how people can dislike constructive criticism; if anything i want to improve, and do not wish to make mistakes (or at least not twice) but learn, get better.
politeness... rarely helps with that.