Serious cases of libel / defamation / character assassination: serious baseless accusations that cannot be resolved through discussion with the author. For example someone spewing thread after thread accusing someone of something reputation damaging without evidence. Note that libel doesn't apply with fictional characters.
Unethical content: content considered to be unethical or illegal in most jurisdictions like child pornography, apology of violence, terrorism or racism etc. Note that Steem being a blockchain, it doesn't fall under any specific jurisdiction, so legality cannot really be determined for sure. Moderation decisions by the community will likely converge stronger on consensus around ethics / morality rather than legality, but legality is still a good hint do decide whether to make a report.