Pierre Uszynski writes:

Significant abuse is that which causes significant problems for the operators: posting secret religious technology, forging prime minister mail, harrassing a member of any number of opposite persuasions, etc...

on that note, check out this excerpt from the remailer-help file of the q@c2.org remailer:

Abuse/Self-Preservation Policy: Abuse of the Q Mixmaster Remailer consists in those uses of the remailer which endanger its continued operation. Please don't ruin anonymity services for those people with legitimate needs for them.