I'm one of the users migrating from Postbox. While I've gotten most of the same functionality that I need, one thing I already miss is the ability to apply tags through a quick-menu. In Postbox, with a message or thread selected, there was a keyboard shortcut that opened up a tag text box, and you could type in the tag you wanted to apply (with autocomplete and you could use commas to add multiple tags) and they would be attached to that email. Much faster and easier than needing to click through the tiny tag icon and select from the list, especially if you use tags a lot and have a lot of different tags.