Currently the places plugin requires two steps to open a new volume on removable media, ie. one to mount, another to open. In Thunar, volumes are mounted when they are opened. There should be a preference in the places plugin so that its behaviour can be made the same as Thunar.
I reorganized some code today to support this feature. Each of the models (system, volumes, user) is now fully in charge of specifying *all* of its own bookmark actions (right-click menu), including a new "primary action" (left-click). The next thing for me to do is to create a "Mount and Open" action.
Completed in rev 3398. Now there is always a "Mount and Open" action on unmounted volumes. There's a preference to toggle whether left-clicking should open the actions menu or Mount and Open, defaulting to the former.
*** Bug 3673 has been marked as a duplicate of this bug. ***