Around 30% of screen in my browser used just to show panels, not questions.
Please, look at screenshot, I drew how can it be improved:
Using a screen resolution of 1600x900, I have space in both the left and the right side, even without using a windows as width as the full screen, but that is not what happen with lower resolutions (as you can see from Paŭlo Ebermann's screenshot).
Changing those sections from horizontal to vertical is not a good idea, IMO. It would not resolve the issue for everybody, and some users would get a worse user interface. (Maybe having those sections in vertical is not a good idea for who uses higher resolutions either.)
If something can be done is to allow those sections (where they are are now) to be collapsible, and let the users decide of they want those sections collapsed by default. Naturally, if the users don't change the setting, those sections should be visible by default, and there would be an element in the user interface that allows to collapse them, or show them again.
It is also true that those navigation links placed on the top are normally not collapsible. You can see, as example, what done on Drupal.org, where the links on the top are not collapsible.
This might work on your screen, but on my one there simply is no space at the left page border.
I think the page width is adapted at a screen width of 1024 pixels ... just what my screen is. If you want to put the navigation elements left instead of on the top, this either has the result that I have to scroll horizontally, or that the horizontal space for the questions will shrink - which mean that you in effect will have less "space for useful information".