The main problem that arises when using the Word HTML rendering engine is that Microsoft Outlook 2010 will not support web standards like CSS or background images. This could very well mean that Outlook 2010 users will be stuck with broken HTML emails for the next years after the release. Below is an image that is showing a HTML email in both Microsoft Outlook 2000 and Outlook 2010:

Microsoft Outlook 2000 was using the Internet Explorer rendering engine to display HTML emails. Microsoft mentions benefits that Outlook users gain by using Microsoft Word as their email authoring tool which include automatic styles and templates and SmartArt among other benefits. It would also not be in the best interest performance wise to use a different rendering engine for the display of HTML emails. (via Email Standards)

The initiators of the Fix Outlook protest website are making use of Twitter to rally protesters from all over the world. Every Twitter user that mentions fixoutlook.org in their tweets will automatically partake in the protest. The avatar icons of users will be displayed on the Fix Outlook website. Currently more than 15500 users have already joined the protest.