This post is not about the left navigation bar being annoying or something to disable. Unfortunately we do not seem to have a post about suggestions to make the left navigation more useful now that community sites actually have them and the users have tried to use them (some good proposals have been made in the Left nav, responsive design, and theming next steps post).
In this question I will try to get my point across concerning how to make the left navigation more useful on a normal network site with a usually very empty nav bar (I am primarily on tex.stackexchange.com). I will not repeat the ideas mentioned in the linked post where the focus seems to be on tag searches, shortcuts etc. Just for emphasis: I also support the idea of the site selector in the nav bar.
While working with the site's new navigation concept I found two tasks more tedious than I think they need to be when navigating the site:
Review. I am not proposing to remove that nice button from the top bar, but maybe you could ease my workflow by removing one click from the process of navigating to another queue. To show what I mean, this is the current situation:
While this obviously works, I seem to remember that a navigation bar has been introduced and this context (review) definitely needs an easier navigation path. Hence I would suggest a review navigation field (as an example for context dependent navigation, e.g. while you are at
*/review/*) like in the following picture:
Identification. This is basically the same idea as @Raphael's post It is now all but impossible to tell sites appart at a glance which I just stumbled upon, so I won't go into detail on that. That said I would think that when scrolling down users with non-sticky top bar should get a clue about the community they are visiting and I think that the sticky left navigation would be a great place for this.