I don't know why this email is getting so much attention. Well, the best reason I've heard is simply that if BillG was concerned that much about user experience, why isn't MSFT perfect 5 years later.
It's a fair criticism, but not particularly. There are many products that have been released since then from MSFT that have obviously invested heavily in UX. Live Writer, OneNote, PopFly, Office Ribbon, Office Communicator, Live Mesh are some of these examples.
The article claiming that the BillG email is a "rant" is silly. I hate to say it, but I've gotten emails just like that one from people who care about product usability: prospects, customers, C-levels, and I've even written similar notes — though probably with a more neutral tone. Frankly, I sometimes even seek that kind of email out. In the middle of development cycles, you get wrapped up in shipping software, and get used to the idiosyncrasies that get introduced during development. Shipping prototypes out to customers or screenshots early in the development cycles can help alleviate getting emails like this from the big boss after you've shipped.