I have quite a lot of different folders in multiple levels, to which all my mailinglists and different mail is sorted. However, opening Geary means that I have to manually expand and click on all arrows to expand the folder tree to see all the folders. 
I think it would be nice if Geary would remember the state of the tree that the user left it, when the application is closed. If no previous state is found, I think it should default to just expand all the nodes in the folder tree. I guess if I as a user have a lot of folders, it is because I like to use them.