Extortion.   "You keep using that word. I do not think it means what you think it means."

Mt.Gox failure to disclosure their identification policies in advance may be shady, hell it may even be illegal (doubtful) but under no possible interpretation of any law is it extortion. Even if it WAS extortion that is a criminal matter.  You can't seek civil damages for a criminal charge.  Even if a judge ruled that MtGox can't ask for your ID you have suffered no damages (legal sense of the word).  You can't seek redress for damages never suffered.

Under the totally implausible scenario that anyone took your case seriously, and you made it to court, and survived motions to dismiss, and ended up winning at best you would get your $100 back (maybe with some interest and no not that 1% per day interest in the lending forum the 1.5% APR interest you could claim as damages).  So in a year you might get $101.50.  Well no you won't but that is under the most asininely optimistic scenario you could possibly hope for.