Mike Torres has an interesting post that's mainly about cross-posting, but he mentions something about being able to flag all posts that link to a particular URL as something you're not interested in.
That got me thinking. Wouldn't it be cool if your aggregator could look at all of your posts and have the ability to group them by URLs they link to. Then you could quickly sort them by link count, and you'd get a good feel for what is hot. For example, over the past few days, if I were to do that, the top two links would definitely be the Microsoft Longhorn/RSS marriage and the new release of iTunes.
Of course, this would rely on everyone using the same link to the actual news source, but some of that could be done by traveling the links to get a chain of links, and find the eventual source. That would be time consuming, but the ability to view what's hot among your network would be very cool.
And that's the key. Search engines like Feedster or Technorati are great for aggregating links across the whole blogosphere, but I've built a network of bloggers that I read and trust the information they post. While I enjoy finding new blogs that I'm interested in (I find probably 3-5 per week), I'm more interested in knowing what people in my network (yes, if I read your blog, then you're part of my network) are interested in and what their opinions are about trends. By being able to aggregate by my subscriptions, I get that.
This is probably a feature that would be better for the big online aggregators, such as Bloglines or Newsgator Online. Most of the URL information could be saved off so that it could be used across users - you just need to know what blogs each user is subscribed to, but you already would have the parsing done. With a client-side aggregator, the parsing and storing of information has to be done on each client aggregator, and the cost of performing that operation is higher per user.
Anyway, this could be a killer feature that could set an aggregator apart from others.