Well as is often said here - anything 3 elders agree on.
Some specifics - sex, alcoholism/drugs, badmouthing the cult or doctrine, being tangentially involved in fraud (happened to a personal friend of mine - he was innocent), porn, blood transfusion, misc bloodguilt, misc crimes (theft assault, etc), joining another church, joining any political group, smoking, voting, any form of military service... I'm sure there's lots more but I'm drawing a blank now.