popularity is on the left side of the post designated by the colored envelops. example yellow means its a hot topic (has over 15 replies in that thread) you can see the"legend" by scrolling down the page. below the list of threads, below the display options you will see a bunch of envelops that designate what all the various popular levels of envelops mean.

members can also use the posting icons to designate what their thread is.

popular viewing is also noted on the right side of the page that shows how many people in general have views a thread.

keep in mind though the views vs reply numbers never match up people outside of psych central can read the boards too with out being able to reply because they havent registered yet and also people sometimes read threads many times before they decide to reply. example when I dont have time to log in I will do a quick read of a few posts while not logged in, I was also able to read the boards before I registered for an account and sometimes I reread threads many times before actually completing my reply.