In Reddit you can approximate this with post flair (tedious to set up, but doable). See http://reddit.com/r/AskScience or http://reddit.com/r/AskHistorians for good examples.
Alternatively: an RSS reader with categorized streams I can follow. Again, Reddit offers this:
http://www.reddit.com/wiki/rss
http://www.reddit.com/r/pathogendavid/comments/tv8m9/pathoge...