- Is there a full list of improvements for 13? Where?
Your best bet is to refer to the website. A good start is...
Any chance of there being native HTML email sending?
This feature still requires a plug-in.
FM Advanced/Pro - http://help.filemaker.com/app/answers/detail/a_id/12530
OS Requirements - http://help.filemaker.com/app/answers/detail/a_id/4701
Adding to Joshua's list of links, the "Installation and New Features Guide" (pdf) at the following link has the complete list you're looking for:
Looks like the web version of FM13 help isn't up yet?
But this does...