Although it's probably possible with a lot of custom programming, I don't really see concrete5 as a good starting point for such a project.

Perhaps you could use reddit's codebase? -https://github.com/reddit/reddit...

The only thing about using reddit's codebase is I want the other features concrete5 provides because the link submission system is only going to be one aspect of the site. I like concrete5 for the event scheduling and forums, etc.

There are some addons for ratings, but upvote/downvote is a little different from that. It's partly about rating, but also organizing the content. I could see it being possible to create some aspects of this sort of site using C5 core features like attributes combined with page lists. If you could find a ratings system that could be modified to track up/down votes that would be a good start.

It is unlike in C5 or any other CMS today that you can recreate something as specific as Reddit without some custom programming. With that being said I think C5 is the best fit compared to say WP or Drupal because with it flexible attribute system that you could extend for the ratings and the page list system that could be used to sort and filter the content... combined with some single pages and some perhaps a few custom libraries anything can be achieved with C5.

I've been recently experimenting around with "UberList" from Johnthefish and I can imagine that in combination with "Magic Data" this Add-on might be able to solve the vote-filtering-issue.

For all the other reddit-ish functionality, I think you can create a lot of it with concrete5, but there will be some coding or spending involved to have it all. But I could not think of any other ready CMS that could do it out of the box...

I think @goutnet's up/down vote addon may store the votes as attributes (please check rather than taking my word for it). That would make it easy to build out from @cahueya's suggestion of using Uber List and Magic Data to filter and sort pages based on the vote attribute.