Thanks for all the replies!

I'm aiming to have a decent enough build as a developer release (non-feature-complete, unstable) by May 1.

Right now, I've been deliberating over a "Rules" feature that I've partially implemented. The idea is that if you drag in an item that meets certain criteria, than it can perform certain actions with it. Move it to a certain folder, apply a certain label, etc. Do you think that would be a useful feature?

The conflict I have now with it is that I have a "compose" feature which (without rules) makes a new entry, puts it in the inbox, and opens it up for editing. There's kind of a UI issue of when is the item actually saved (and ready for sorting).