Ignoring the little stuff, Freelancer has so far revealed to me three primary negatives: $30 minimum bids, the inability for Sellers to ask me questions before placing their first bid, and the whole way they suddenly migrated vWorker without advance warning. Not sure yet about their stance on arbitration, but I'm not holding my breath.
oDesk has a lower (no?) minimum bid level, but makes it clear up front that they do not protect fixed-price jobs, i.e., no arbitration at all. They do guarantee hourly work, but that doesn't really help if some work was done, but the finished product is unsatisfactory/nonexistent. Still, it may make sense if I keep all jobs tiny enough and essentially "self-insure".