Oh yeah, I forgot. Besides having the portal list sorted by popularity, you could also watch it's
change in popularity by adding a small arrow next to the thread title. If yesterday's popularity < today's popularity (or an hour ago, or five minutes ago, or whatever), then display an up arrow (in green?) next to the thread title. If previous popularity > current popularity, then display a down arrow (in red?) next to the thread title.
Some examples:
http://dynamic.nasdaq.com/aspx/majorindices.aspx
http://www.imdb.com/chart/