There is a long-standing approach that argues most people can be grouped into issue publics, topics they care greatly about. I'm skimming a fairly new book entitled Moved to Action (with a long post-colon subtitle) that examines what motivates people to participate in politics, especially when inequality plays a role.
The book is broader than this, of course, but I'm using this as a kickoff on the topic of issue publics from a news media and what people know perspective. The cult of cable news kinda feeds the sense of issue publics in its continuous coverage of a handful of topics. The cable talking heads create, nurture, and maintain these issues. Then again, that's basically journalism's job -- to identify what is news and push it higher on the agenda.
In terms of political knowledge, you might suspect that the more issues one cares about, the better one might do on tests of knowledge. Not necessarily. There is breadth of knowledge, and there is depth of knowledge. These two combined often create a cognitive complexity measure, a topic for another day. Depth of knowledge in a few issues is considered good, but so is breadth of knowledge -- the ability to discuss a number of topics intelligently or at least to care about and understand the basics. Some worry we are narrowing our knowledge base thanks to cult cable news and talk radio, that people know a few topics that matter most to them (or they've been told matter to them, over and over). I'm not so worried because people have always has tunnel vision in terms of issue relevance. For politicians and political strategists, it's a matter of pushing the right buttons. For journalists, it's covering how those buttons get pushed and whether they matter.