1. Why do many historians regard the printing press as the most important invention of the past millennium?

Answer 1

1. There was a rise in the literacy rate, once there was a reason for many people to be able to read. Until printed materials became more widely available, literacy was pretty much limited to nobility and priests and monks.

2. People's ability to read the Bible and think for themselves (not be completely dependent on priests and monks to tell the populace what God's word is) was a large factor in the Reformation.

Here is a good look at this:
http://www.learner.org/interactives/renaissance/printing.html