Visits versus visit duration

Let's plot two metrics against each other in the Audience Overview:
Visits vs. Avg. Visit Duration (diagrams below).
Why look at the number of visits versus visit duration? Well this comparison might highlight specific styles of site engagement.  Have a look at the following report from a low traffic site...

Three distinct kinds of site involvement are evident:

1. Involved readers, high engagement; a user who spends a long time reading through a small number of pages (circled in red).
2. Skimming or surface engagement; a user who visits a few pages quickly but stays only a short time (circled in orange).
3. Sticky or correlated readers; the longer the visit the more pages they view (circled in green).

Figuring out who these readers are is the tricky bit.

Note that you may not be able to make the same conclusions for a high traffic site however as different styles of reader behaviour will disappear into the averages.