Newer versions of OS X have opted to disable a feature I use quite a lot, in order to retain consistent with behavior on iOS. That feature is the ability to hold down on a keyboard key and have it repeat until you stop pressing. To enable repeating keys in OS X yosemite enter the following command in Terminal.app.
After pressing enter you’ll need to quit and relaunch any programs that you want repeating keys enabled in.
I’m not sure when it happened, but this is a change from the previous method of enabling repeating keys which, as of OS X Lion was: