As a moderator on other forums, I find trolls are easy to deal with. Pull them aside, give them a warning, and it's a rare day you ever need to ban anyone except spambots. The real hard part is when established community members actually go one step too far. I've seen it happen; a legitimate gripe between two people escalates out of control and things are said that cannot be taken back.
I've also had a full-scale uprising on one forum for promoting a moderator without first "consulting" the community (I was head moderator on that forum, so I had authority to promote and demote other mods). It wasn't so much my decision, since everyone agreed we needed a moderator who lived in a European time zone and the choice of members was dead obvious, but rather that I hadn't respected the community by having a full and forthright discussion before the promotion. One of the most surreal and enlightening experiences of my online life, in hindsight.