In some cases, transparency/translucency is awesome, and makes for a really great experience. For some people though it teams up with the new default fonts in Yosemite to make things look a little blurry. It might look nice in a demo, but in real life it's making things difficult for some people. Luckily there's a way to disable this and save yourself a trip to the eye doctor.
Go to System Preferences > Accessibility, and click on Display in the options on the left hand side:
System Preferences - Accessibility
There is a checkbox for "Reduce transparency" which gives you full-on opaque menus in Yosemite, not as seen in the Keynote, but certainly better than feeling like suddenly your Mac has gone out of focus!