An important part of the question here is are the subquestions of what should be regarded as a "common carrier"-like thing (a category which probably includes email traffic and public Internet traffic at the TCP/UDP level), and what should be tolerated on those services, and what should be regarded as not similar to a "common carrier" (which clearly includes, say, the official Wikipedia version of a Wikipedia page or the non-advertising content of an issue of the New York Times), and what should be tolerated in those venues. Part of the problem of Reddit is that it blurs the lines between the two categories and sends vague or contradictory messages about which side of the line it's on. Things that make Reddit look more like a single publisher or "community" include:
* the single /r/whatever namespace, which was meant to be used as the marker for the canonical or default reddit on a particular topic (and is still used by users for that purpose, whatever disclaimers Reddit may want to hide behind now)
* other technical features like Reddit gold which likewise encourage the perception of Reddit as a single platform
The just-a-platform argument is hard one for any message board or single-host/owner, single-login cluster of message-boards to sustain. Reddit's even more poorly placed to sustain it than others. Twitter, for example, is much better placed to do so, even though it's a single-owner/host, single-login publishing system.
* the single /r/whatever namespace, which was meant to be used as the marker for the canonical or default reddit on a particular topic (and is still used by users for that purpose, whatever disclaimers Reddit may want to hide behind now)
* the partly-shared moderation across subreddits
* talk about "the Reddit community" and "Redditors" (wait, it's not a community but rather "a community of communities"? http://www.redditblog.com/2014/09/every-man-is-responsible-f... ? Excuse me while I pick myself up off the floor), the Snoo branding and individual subreddits' customisation thereof, the promotion of shared Reddit meetups http://www.redditblog.com/2014/05/global-reddit-meetup-day-i... , and in general the consistent effort to promote a common identity for "Reddit" and "Redditors"
* other technical features like Reddit gold which likewise encourage the perception of Reddit as a single platform
The just-a-platform argument is hard one for any message board or single-host/owner, single-login cluster of message-boards to sustain. Reddit's even more poorly placed to sustain it than others. Twitter, for example, is much better placed to do so, even though it's a single-owner/host, single-login publishing system.