The best argument against Cryonics as far as I'm concerned is economic: It's a self-negating prophecy. Once the technology exists to revive frozen people (I don't have any problem believing this will happen some day) there will be no market for cryonics - in this future, why bother signing up for cryonics when you can get revived at "death" or otherwise forestall it - and therefore no income for cryonic companies. Who is going to maintain the freezers or revive you? Chances are everyone who cares about you is either dead or in a similar predicament.