As Mr. Dog said, "mark above as read" would be a very, very welcome. Often, in a feed or category with hundreds of unread articles, I scroll down to the first headline I find interesting, then want to mark as read only the articles whose headlines I skipped over. Google Reader permitted this with a Chrome extension; hopefully This Old Reader will add the feature by default.

I agree but an alternative is what "reader plus" gives you, an option to mark all items above or below the current item as read. You can do that as the default by turning on the "scrolling marks as read" and "selecting does a scroll" but it's not necessarily something you'd want to have on by default. It does save a lot of time though for feeds with lots of posts where you tend to only read a small portion of them.