My new killer application would be TiVo for my RSS feed. Actually there's one part of TiVo that my RSS reader needs - ratings. There are a number of RSS feeds that contain both useful and not-so-useful information. Today it is an all or nothing proposition. The reader either gets all of the feed or none but can not easily say "more like this".
Actually the full ratings system would be great and would allow a newsreader to find "suggestions" ala TiVo. Long before that there could be a great application for RSS feeds of job postings. Suppose I want to see job postings in the Seattle area. I would subscribe to a feed for all jobs in the newspapers in the area. When it comes to postings for drivers or cooks which I lack the skills for simply mark the first few as "thumbs-down". As the system gets trained it learns that I don't want to see ads related to sales positions but sales engineers or sales support postings would be interesting.
This could be done on the reader side or on the server side. Even in middle ware could be developed to handle aggregating the feeds and then disseminating posts along with an added tag line for "thumbs up" and "thumbs down".