Did they remove the number of emails in a folder from the toolbar in Yosemite's Mail.app?
It appears to be a bug and has been reported to Apple.

A discussion on the Apple forums mentions this same problem, OS X Yosemite mail stopped showing total inbox message count:


  This appears to be a bug. It is triggered when you view a smart folder - once you do, the count in the title bar will disappear until you restart mail.
  
  I filed this as a bug with Apple, so they're aware of it. No ETA for a fix though.