I wouldn't pay for a hosted/webapp RSS reader, however, unless it defied the laws of web app design and cached all the data from my feeds before I started reading, switched between feeds and articles with imperceptible lag and no network access, and had the best UI ever made with HTML5. (Likewise, I wouldn't pay for a native app that didn't sync read/unread state across devices.)
In busy weeks, I 'read' HN via NetNewsWire (yeah I want something better, but that doesn't yet exist). It takes about 4 seconds, plus the time for any articles that you actually click and read.
Bonus $$$: I would pay for an RSS reader that let me click stories in the HN feed and, instead of opening them, add both the src article and the HN comments page to Instapaper.
In that case, It'd have to be significantly better than e.g. Google Reader (which got a little crippled unfortunately). Personally, some of the most important features would be, that it has colour schemes (e.g. like the Vim Solarised scheme) and allows offline caching/easy, good e.g. pdf export.
However, I have to admit that a decent mobile (iOS, WP7, etc.) Application would be of much greater interest to me, as that is where the advantage of a RSS reader significantly comes into play due to the much more limited screen estate.
I've been waiting for a service that relied on published open source code, but still had a hosted option. I'm much more likely to pay in this circumstance.