A quibble:
You say

“We’re also not just ‘pro-EU’ – we won’t include posts if you just think the EU is some sort of conspiracy but we welcome a critical perspective”

It’s a bit “dodgy” to my mind to arrogate to yourself the right to choose which “critical perspectives” to publish.

In the past I also got interested in aggregation and I built this site
http://mappademocratica.net

We collect as a group addresses on Diigo and then I built an app for aggregating a subgroup of them (the set of Italian democratic circles).
The last version of the app was written in php with the use of CakePHP framework and SimplePie library (a very early version was built using wordpress and a plug-in).

The app remained in beta, because I got so disgusted with the Italian Democratic party that I was unable to continue working on it and donating them my time… so I left the project… It was a great relief but I’m considering at least to correct a bug in the algorithm which reads and saves the syndicated material… the problem is my lack of will…

All of this for saying… why not adding a “digg” style content selection feature?

I was thinking to add something as that also in my app since the very beginning even if I didn’t actually did because of lacking of time, I think it could be not so difficult, even if it depends on what kind of people you want to allow to vote: i.e. only people who subscribed the site or even anonymous ones (in this case you need to allow only one vote per IP).