I just discovered that you can add any RSS feed to the WordPress.com Reader – and not just blogs hosted on WordPress.com.
While logged in and viewing the Reader, simply click on “Edit” in the right-hand panel next to “Blogs I follow” (direct link: click) – at the top of the list that opens is a box where you can enter any site/blog URL to follow. It doesn’t even have to be the feed/rss URL, the normal URL works just fine.
Posts from non-WP sites will be missing WP’s “social” features such as the Like button and direct commenting of course, and I for one still prefer a dedicated reader (I’m using G2Reader) because it allows me to categorize the feeds into folders. Makes it easier to digest large numbers of subscriptions. 🙂 But if you’re only a casual reader that consumes blog posts more like a Twitter feed, this probably works well enough.