Adding new feeds is difficult.
The UI is jammed into the lower left corner - an area of the screen rarely used for functional UI. The folder selector falls off the bottom of the page or becomes very small. The plus icon is missable and not intuitive.
Suggestion: move the add section to the top of the page. Make the UI explicit - perhaps it could say “Add” or “Add new feed.”
This is what I see in the lower left corner: https://www.dropbox.com/s/jhsjz0a4gwnar4l/Screenshot%202015-01-12%2013.25.02.png?dl=0
Or provide other methods for adding new feeds: add by email - email a blog post in and add the most popular feed from same domain. Chrome extension - click extension button while on blog post, see feeds from same domain, add. Bookmarklet. Etc.
I presume one reason that adding isn’t as easy as it should be is because you don’t want people to overload newsblur. That’s reasonable, but I’d opt to make the UI clear and easy to use and deal with the overload behind the scenes.