Must add how impressed I am about this developement too. Feasibly, I could delete about half of my viewschemes, due to the bi-directionality.
For absolute total convenience, how about adding a drop down "Sort by" list somewhere in that top bar. You could save your own presets and then would have to complete flexibility to change the order of the list on the fly. You wouldn't necessarily even need the sorting options in "custonmize current view".
I'd also still like to be able to see the "album thumbnails" in the file list at the bottom too.
And I absolutely second Tolga's suggestion for having the buttons along the top act to filter the currently selected tiles, rather than the current way of restting the list. And this canbe done by combining the breadcrumbs and buttons.
The way I would do this is to start of with just one button at the left, set to the user's defualt preference (eg "Genre"). When someone clicks on one or more tiles a second button pops up to the right of the first button, labelled something like "view" and the drop down arrow (just like the one on the tiles at the moment). You then use that drop down menu to filter the tiles to the next view item, and the process continues on the same logic.
Then say you want to backtrack and change something, you just click back to one of the buttons and if you select new tiles inside that view, any buttons that were to the right of it disapear and the next "view" drop down menu button appears instead.
Does that make sense?
One final thing. Back and forward seem to be much more important in the new library style. I know space is a problem, but I'd put them in that top bar if I were you. Or at least make sure that they are by defualt, when you install the program, in the top and bottom toolbars. When I installed the software those toolbars were empty, and I only managed to populate them because I already knew they existed. For new users, there needs to be a much more obvious clue.
Sorry to go on
Great work though!