It would be nice to have some way of making search operators discoverable.  Perhaps we could do something similar to gmail, where when you focus in the search box, a small UI pops up that gives you some options of specific fields to search on.  As you type in the specific text entries or check checkboxes, it would just fill in the search text for you.  This way you can just start typing if you already know the operators, or you can let Geary "build" the query string for you.