There are two conflicting designs for RSS aggregators. On one hand are email-style three-pane readers, typically found in desktop aggregators such as SharpReader, RSS Bandit, and others. On the other hand are blended-feeds aggregators, like Userland's Manila and Radio, which take the latest news from all of your subscriptions and blend it into one big time-sorted page.
Dave Winer strongly advocates for the blended-feeds style. When I first heard him rant about this, in person at a Berkman Thursday meeting, I didn't know why he cared so much. I had used both styles of aggregators, and I thought the difference was really just an issue of taste.
That was when I had a small number of subscriptions, around 30 or 50. Now that I have over 200, there is a major difference. In a three-pane aggregator, when you can't read all the news, you skip feeds that aren't often of interest. In a blended-feeds aggregator, you skip postings that are older, or have boring-sounding titles and no pictures.
While the latter seems like a more arbitrary way to prioritize your reading, it actually has some advantages. If you have a big set of feeds that you kinda-sorta read, but aren't devoted to tracking every last post, it works better. With a three-pane aggregator, there is some point where you just give up on a feed—it gathers messages for months, but you can't remember what the hell it is, so you never click it. After a while you can't remember what it's there for and the 312 unread posts makes skimming it too intimidating. Unless you regularly schedule time to organize your subscription list, it just sits there. (Maybe we need a way for computers to emit smells, so you can actually notice it rotting?)
On the contrary, with a blended-feeds aggregator, you'll occassionally see a message from that feed. Either you'll like it and keep the feed, or you'll realize what crap it is and unsubscribe altogether. You're less likely to maintain a meaningless subscription. And the subscriptions you do maintain will all be given a fair shot: your existing preferences for a certain core set of feeds don't preclude everything else to the dustbin.
There's a reason this advantage isn't evident to new aggregator users: it depends on a particular memory threshold. For a while, it's easy to (implicitly) rank a bunch of feeds in your head. Some you check every time they go bold, and that's great, because you never miss or wait for news from the sources you care about. Others you check at specific times, like the comics I read every morning. Others you might save for when a certain topic crosses your mind, or simply for when you're bored.
Up to a certain number of feeds, your mind is capable of telling you when to read what with pretty good accuracy. But at a certain point, the accuracy starts falling off. It becomes risky to subscribe to a new feed because either (1) you will love it and it might push off something else you care about, or (2) you'll not love it enough at first and it will fade into the abyss. But what if it starts to be interesting again? You're not giving it even partial continuous attention, so you miss it.
That's right: in an aggregator that seems designed to help you read every post, you actually miss all sorts of interesting posts. In one that seems to make it difficult to ensure you've read all you care about, you're relieved of caring—and in the process, you're more flexible and open to exploring.
Nonetheless, both approaches have their advantages. Thus the problem is that you're forced to pick one or the other for your whole aggregator. No fair! I have feeds from friends, colleagues I respect, serial publications I want to follow every bit of. I also have feeds from a gazillion social software nerds where I often want an update but can't possibly follow everything.
So my aggregator should let me put the most important feeds on watch lists. It should make sure I've read every post on my watched feeds. And it should let me quickly skim everything else in the more flexible, more efficient blended-feeds format.