One thing annoying in Lion is the popup appearing when you long press a key (similar to the one present in iOS showing special characters).
I found a way to disable this by issuing a simple Terminal command :

defaults write -g ApplePressAndHoldEnabled -bool false

Replace false with true in this command if you want to bring it back, you might need to exit and relaunch an app for the parameter to take effect.
If you want to discover more parameters, don't hesitate to dig into built-in properties, that's the way I found this tip (the -g flag tells to lookup in the globaldomain)

defaults read [-g]