Griefing, trolling, hacking, exploits, and threatening other members. Most of the others can be dealt with on the spot, but the very first one shows why it's also helpful to make redundant backup files of the world. If you have a tight-knit community, I guess you could also make it a ban-able offense to be making constant drama or harassment issues within the server, but that's for extreme cases in my opinion.
To prove that point, had to ban a guy who was recommended to join our private server who was otherwise a standup guy. It started as basic insults but then slowly escalated to borderline harassment; It was very clear it was influencing the members of my little server though, so after a failed warning and having it ignored I promptly banned him on the basis of having a lack of basic human courtesy. Only time i've actually banned someone so that's about all I could even offer.
