To date, I have been enjoying Yosemite's overhauled design, including its translucent effect that lets the background colors bleed through some windows and menus. Of course, your opinion may differ. If you find Yosemite's translucence distracting or annoying or both, there is a way to turn it off.
1. Open System Preferences.
2. Select Accessibility.
3. Click on Display from the left panel and then check the box for Reduce transparency.
(Yosemite uses the more general term "transparency" here rather than the more accurate term "translucency." And it claims only to reduce the effect, but to my eye it disables it altogether.)
For more Yosemite-related tips, please see CNET How To's guide to Mac OS X 10.10 Yosemite.