I think the continuity features like automatically being able to load a Safari tab that I've loaded on any other Apple device is great. I thought Spotlight wasn't very good until I used it with a SSD. Many of the Yosemite features will probably become better over time as they mature, too.
It's a system for Spotlight plugins on Yosemite. It works similar to Alfred, LaunchBar or Quicksilver. It's open source and fairly simple to write your own plugins for the system (in Python or Javascript).
