The technology isn't the hurdle here, it's the prestige. The academic publishing world is all about who you are, what institutes you belong to, etc. To put it bluntly, a Reddit-like journal site just wouldn't have enough "cred" to attract decent content.
Indeed, get someone like MIT to do an OpenJournal to complement the rest of their OpenX initiatives and you might start getting buy-in.
The problem is that the system is broken - publish n papers => you're better than before (ignoring the content of the papers of course). Academics are encouraged to publish (and re-publish older stuff with a slight tweak) to meet publishing 'targets' handed down by government.
This model feeds into the Elsiver etc closed publishing as academics are forced to compete to earn their stripes - so now we have a model that encourages re-publishing crap and then locking it behind a paywall, but when I ask PhDs if they think this is fine, they don't see the problem :(