In view 'Thumbnails', subfolders are displayed. Though, usage of the folders is a bit difficult/cumbersome since you do not see a folder name. The tooltip helps, but that is cumbersome.
O.k., there is view 'Thumbnails + Filename', but perhaps a solution for folders in view 'Thumbnails' could be found. E.g. displaying the folder name inside the folder or making the folder a bit smaller and displaying the name underneath might be a solution.
Please note that I'd consider priority of this issue as low.
0.51: View: Folders in view 'Thumbnails only'
Moderators: helmut, XnTriq, xnview