In his post, Interface Design is Copywriting, Joshua Porter says:

I humbly submit that there is no single element more important to your interface than the copywriting. There is nothing that makes or breaks a positive experience more than the simple set of words that you choose to communicate with. In a world in which we have to simplify as much as possible, nothing matters more than the small vocabulary you end up with in your final work.

Amen to that.