After putting together Simply Structured and years of hearing people gripe about the dearth of style in feed readers, I've realized my biggest request for NetNewsWire, or any reader for that matter, is customized styles for each feed. Even more, I don't just want the end-user to be able to customize styles, but for the author to be able to push styles alongside their content.
Styles on a feed-by-feed basis isn't a huge stretch, especially in NNW where style packages already exist, but pushing styles with your feed is something RSS and Atom don't support. But so what? In the early days of HTML, Netscape Navigator went beyond the HTML spec and added unsupported styles that developers wanted and the web is a better place for it. I'm not advocating for every feed reader to require its own custom flavor of RSS, but if they could add one line that was easily ignored by other RSS parsers, it would make for a fantastic experience. If we're already designing custom versions of sites for our iPhones and other mobile platforms, why not feed readers too?
You could argue custom styles unnecessarily complicate a feed reader, but I don't think it effects how most people consume feeds. The primary benefit of feed readers is having a device that notifies you when there is new content. The second largest benefit is having all the content in one place. Individual styles don't diminish these facts and help bring some individuality back to the web. In a time when we all have custom homepages and visit fewer and fewer sites, it'd be nice to inject a bit more personality into our daily lives.