Inability to follow your favorite topics sans others from your favorite sources seems to be a fatal deficiency in the web ecosystem! Yes there is Twitter, but Twitter only lets you follow a source ( person / group / entity ) in toto and not selectively.

Let me elaborate! Imagine you like the way Frank (an imaginary person) relates to 'Politics' but don't care about his numerous rants about 'Technology', 'Sports', etc. Now, ideally you should be able to follow #politics from @frank i.e. politics@frank or frank@politics, which ever sounds more idiomatic, but such an option doesn't yet exist on any web-service / web.

The closest thing you can get to this pattern is when Frank writes a blog and categorizes / tags his posts with a provision to subscribe to the RSS feeds of those categories / tags. Many blogging solutions offer such RSS feeds and one can easily subscribe to such feeds from his favorite reader.

However, if you didn't follow the scheme detailed in the previous paragraph, don't be intimidated! You are not alone! It may be a cakewalk for the web savvy but not for the rest of us humble folks on the web.

Why not emphasize categories / tags everywhere on the web and put a simple 'Follow' button on each one of them? This way we can follow what we love more selectively and reduce the information load on us! :-)