I wrote this in a comment to David Ascher's posting about SOGo: Thunderbird-inspired and Thunderbird-compatible Groupware.
One of the great advances of the desktop rich applications was the compound document. Mixing text, images, interactive charts, active diagrams, busy/free displays with a simple drag and drop. The machines of the time were a little underpowered for this task but that should not take away from the advance. The compound document frameworks were also very difficult to program to. Mostly the power software houses like IBM, Microsoft, Borland, Lotus, Claris, Taligent, etc did it. The underpowered houses could not and so created data and display islands.
Another of the great advances was the expandable shell. The first one was OS/2's Presentation Manager. Then came Windows 95's Explorer. Linux had Nautilus. It is a very compelling idea to have a single tool that integrates browsing and searching hierarchical collections of data. The shell frameworks were also very difficult to program to. Mostly the power software houses like IBM, Microsoft, Borland, Lotus, Claris, etc did it. The underpowered houses could not and instead presented other independent hierarchical displays.
We need to go back to these ideas and implement them now with the more powerful machines and high level languages and toolkits. I really don't want Thunderbird to be so deep so as to enclose these tools and data repositories. I want a lighter touch. I want Thunderbird to provide interactiveness that is not currently available via the browser. Thunderbird needs to be a super-browser. I want it to show the way to where browsers need to go.