Search criteria is an option that's available whenever you look for files and folders through the search field in the upper right corner in Finder windows. You used to be able to click the little Plus button next to the Save button that appears once you start your search, but in OS X Yosemite, it's just a greyed out minus button that's no help at all.
Finder search criteria is hidden in the Actions menu
To bring back search criteria, open a Finder window, then click the Actions button in the title bar. It looks like a litte gear. Now choose Show Search Criteria.
Search criteria lets me turn a 47-item result list…
You'll probably see a bunch of search options show up in your Finder window. If you want a less cluttered window when you aren't performing searches, just click the minus button next to Save. The minus turns into a plus so you know the options are still there — just click to show them again.
…into a manageable 9-item list.
The upside of showing search criteria is that you can trim down search results dramatically. If you know you only need to see Pages documents about Apple Watch created in the last two weeks, search criteria lets you do that instead of showing every document, email, and Messages conversation where you mention Apple Watch.
The downside to Yosemite's new way of handling search criteria is that you'll have to reenable it for new searches in other Finder windows. That's kind of annoying, but better than not having the option available at all.
How's that for finding the silver lining?