The other day, I upgraded my wife’s MacBook Air to OS X Yosemite. Immediately, her first concern was that the UI text was harder to read. This is, in part, due to Helvetica Neue replacing Lucida Grande, but also because Yosemite’s LCD font smoothing seems rather harsh on non-Retina. Fortunately, a Terminal command makes for a crisper and easier-to-read display:
defaults -currentHost write -globalDomain AppleFontSmoothing -int 2
Log out and back in and you should see some improvement in text rendering in the Yosemite UI.