In the same way when was introduced two years ago with OS X Lion, Handoff in Yosemite will be limited to Bluetooth 4.0 LE enabled Macs reports MacIssues:

• MacBook Air (Mid 2011 or later)
• MacBook Pro (Mid 2012 or later)
• Retina MacBook Pro (All models)
• iMac (Late 2012 or later)
• Mac Mini (Mid 2011 or later)
• Mac Pro (Late 2013 or later)

This is a technical decision rather than some kind of planned obsolescence. Handoff in fact uses low energy Bluetooth to communicate between devices even if not connected to a Wi-Fi network.