My first experience with a pre-release build was back in 2009, with Windows 7 RC.
I noticed something: There was an expiration date for it.
Later i learned that the expiration is called a "Timebomb" and prevents the OS from working properly after a certain date, by deactivating it's license and disabling some important features.
But what exactly is the purpose of it?
Linux alphas, betas and RCs don't expire, OS X ones don't expire as well, as far as i know.