The debut of the Ribbon UI in Office 2007 was the tipping point where I started to look at other platforms and other productivity suites. It made no sense at all to me to switch from literally decades of convention in the UI to something that to this day leaves me frustrated every time I use it (probably because it's not often).
The Ribbon is why I now use a Mac, and don't use Outlook or any Office products.
Windows 8/Metro is going to be another such pivotal moment. It's so different, I wonder how many people are going to say "wow, maybe I ought to look at a Mac / Linux if I have to relearn everything anyway."