Apple has disabled key repeats by default in Yosemite. To enable, just run:

defaults write -g ApplePressAndHoldEnabled -bool false

In case you don’t like the delay with key repeats, you can remove it by running:

defaults write NSGlobalDomain KeyRepeat -int 0

and of course disable the annoying spell checker

defaults write NSGlobalDomain NSAutomaticSpellingCorrectionEnabled -bool false