A big thank you to Kevin White on clearly explaining this change....

Only Macs with AES-NI (Advanced Encryption Standard New Instructions) will auto convert to Core Storage when you upgrade to Yosemite. 

(Apparently some older MacBooks cannot use hardware encryption, so making it easier for them to enable FileVault would be the wrong thing to do.)

Yosemite uses a volume format known as Core Storage. It is the enabling technology behind Fusion Drive and FileVault.

Core Storage is a reliable, high-performance volume format. It provides increased crash protection, ditto blocks for metadata, copy-on-write B-tree catalogs, in-place transformations for backgrounding the disk encryption used by FileVault, and intelligent block-level data migration used by Fusion Drive.

When you install OS X Yosemite, existing HFS+ volumes are converted to Core Storage. Using Core Storage allows Yosemite to improve storage reliability, and makes it easy to enable FileVault without requiring a reboot.

This command will tell you if your CPU supports AES-NI.

sysctl -a machdep.cpu | grep AES