The scourge of low quality content has Google putting more and more pest control into their algorithm. That has started killing off the traffic to actual quality sites.

Most notably, it's really taken a hard hit to MetaFilter[1]; the site had been funded by a $5 fee to join (post/comment) and Google Adwords on AskMetaFilter[2]. Now they've had to lay off several moderators which contributed to the sites high quality.

[1] http://metafilter.com [2] http://ask.metafilter.com

From the comments: Google searches 'Panda' updates. More info on Panda here - http://searchengineland.com/metafilter-penalized-google-1921...