OS X Yosemite was clearly designed to look best on hardware like the 27" iMac with Retina 5K display. If you are like me, you don't own a Mac with a Retina display yet. If you have upgraded to Yosemite, you might find text is a little harder to read than it was on OS X Mavericks.
This is how text appears under the default settings
Make one small change and this is possible
Go to System Preferences / General and turn off "Use LCD font smoothing when available." Log out and back in for the change to take effect.