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.
defaults write NSGlobalDomain ApplePressAndHoldEnabled -bool false
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:
defaults write -g ApplePressAndHoldEnabled -bool false