Highlight Active Button in Yosemite I usually work without using the mouse too much. 
In Mavericks the active button (the one that's pressed using the spacebar) had a nice blue outline. Any way to get that back? 

In the screenshot below "Don't Save" is actually active and pressing space presses it. also Tab-Space presses the cancel button. 



Old UI:
This used to work in every application for me until I installed Yosemite. Now, It only works for certain applications. The way around this is to use the 'command + (first letter of the button text)'  For example, if you want to choose "Don't Save" then 'command+d'.