This one's for those of us who are used to RISC OS (or have used ROX-Filer).
Created attachment 649 Implement the new-window-on-left-click option.
As an extension (again, something normally found on RISC OS and implemented in ROX-Filer), the window could be closed on activating a non-directory object with the middle button. I've not (yet) successfully implemented this, though.
For consistency, we use middle click to open new windows. This works on both the view as well as the side pane and the path bar. "New window on left click" would need to implement the same consistent behaviour, which would be rather confusing for atleast the tree/shortcuts pane and the path bar.
I disagree about the path bar, though: different kind of object, looks somewhat like a tabbed box, middle-click just wasn't obvious at all there to me. You're /probably/ right wrt the side panels, but I tend to view them as alternatives to the path bar... BTW, the buttons in the location bar (toolbar) aren't middle-clickable. (Again, here, left-click should probably always be same-window.)
Right, the toolbar is another story. Mainly because it's not that useful to justify the addition of a new GtkAction class, just to get toolitems with slightly different behaviour. For the path bar: You can always right click and discover the 'Open in new Window' menu item. Thunar is not meant to be ROX 3.0, and ROX development doesn't stop because of Thunar. So there's always ROX which offers "uncommon" options (common for RISC OS, but not consistent with other applications on todays Unix/Linux desktops).
Moving to 0.4.0rc1.
This is something for ROX, not for Thunar.