I couldn't find any keyboard shortcuts to add filters and this functionality would make browsing my images much easier.

Ctrl+N = filter by name

Ctrl+T = filter by tag

Ctrl+E= filter by rating

Ctrl+D = filter by date

Could there also be a a shortcut to show/hide the thumbnail bar too?
EDITED
Found thumbnails bar shortcut, it's Ctrl+B.

I am a bit reluctant to add a bunch of shortcuts like that as it is quite a scarce resource (Ctrl+T is already used to tag an image by the way). What about adding a shortcut to popup the "Add Filter" combobox?

Your remark regarding the shortcut to toggle the thumbnail bar is interesting: it made me realize the only way to discover this shortcut is through the "Configure Shortcuts" dialog. I should append the shortcut to the tooltip.

I'll settle for the "add filter" pop-up combo box shortcut key.