Even so I didn't see anything that couldn't be done by placing the tags above the column headings -- whenever the tag browser does anything complex, you pop up a dialog anyway -- I think you could eliminate that whole column on the left.
Totally different UI suggestion: yesterday I spent a couple of hours looking for custom columns -- I knew you talked about it in the video, but it sure wasn't obvious -- putting it in preferences is not intuitive. I would suggest that instead when you right click on a column heading, one of the popup menu items should be "add custom column" which would add the custom column to the left of the column you clicked with perhaps a dialog to specify what goes in the custom column, etc.
As I think about my difficulties with the GUI UI, I realize that to me the calibre display is reminiscent of a spreadsheet, and I expect the Excel conventions to apply -- some do (like sorting columns by clicking on the heading), some don't -- it can be very frustrating. I don't imagine that I am alone in this "excel-itus"
).