Most important MacOS folders to move from SSD to spinning HDD, and how to move them?
For my Hackintosh SSD-HHD "separation" purposes, I moved my /Users directory to be mounted directly on my (spinning) HDD partition.  Details here.

I view symlinking /Users to be generally risky for MacOS.

Haven't yet moved my /Library mount; not certain merit of doing that (eg: hard to gauge /Library disk activity).  Would be great if someone could detail the default system log paths for MacOS (I'm running Lion) to help sort this out.

In any case, my /Users dir definitely gets hammered with lots of write IO + storage capacity (both things not optimal for Flash-memory-based storage), so definitely moved that off.