Naturally, the first thing I'd try to do with Pipes is to mock up something like Eduglu - taking feeds associated with a group of individuals and merging them into one place. The concept, from an educational standpoint, would theoretically make it feasible for students and/or teachers to monitor relevant online activities for an entire class, without having to hunt out each person's various feeds.
It's not quite there yet, though. Currently, the concept of Filters is rather borked in Pipes. If you're dealing with a homogeneous set of feeds in a Fetch, it works fine, but as you add different kinds of feeds, the parameters for filtering go all wonky (is it filtering based on Category? dc:topic? Title? Description? Something else?)
I've mocked up a too-simple version of what Pipes can do with a set of 5 individuals' various feeds (blogs, Flickr, del.icio.us, Twitter). It's essentially unusable as an Eduglu, since there is no way to properly (and effectively) filter and/or prioritize items that are aggregated. And the display seems to be rather simple (title + one line of teaser).
Here's what the Pipe looks like from the inside:
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And the output looks like this:
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I'm going to be putting some more thought into what an interface might look like for defining groups, pulling in an individual's feeds, and displaying the aggregation. This is starting to feel like it's going to need some pretty high-end UI work on top of some pretty flexible plumbing to be able to do this effectively.