I dunno about those folders being buttons. Kinda inconsistent with most UIs.
BUT - I do like the idea of streamlining and modernizing the interface. The current interface still looks like a Windows 95 app, and so much has happened in UIs since then.
One application that really has my interest form a UI perspective is Windows Live Photo Gallery. Yeah, it's just for pictures, but the hierarchal tags really works well, and now I'm wishing such a tag system were available for all kinds of stuff.
Not the flat tags from Gmail, mind you - those become totally unmanageable when you have too many tags. I still like to group the tags in a hierarchy so I can hide groups of them that I'm not using at the moment.
In fact, it seems Pmail already has a type of support for a type of tag, with the link folders. But that should probably be expanded to act more like tags from other programs. The link folders are a very good start for email organization.
Very nice image, good start.
Another wish list item for Pegasus Mail: Indexed search. Computers have plenty of storage for indexes now, and they're rapidly becoming the norm. That, plus put a search box somewhere in the main UI instead of hiding it underneath menus.