I'm interested in seeing where this goes, but ultimately, in addition to the thickness issue, you may find that none of these disks spin slower than 10kRPM and therefore put out a lot of heat, and also take a lot of power.

This would be really interesting to see followed through as a potential source of disks for external enclosures, or for more well-ventilated modular macs. Especially those PowerPC boxes that'll work with HFS + formatting.