So Reddit's a publisher (because most of the subreddits have various on-topic rules for posting) and my newspaper's a platform (because their rules for posting comments are what's prohibited).
One of the problems with trying to use an actor's own policy to distinguish between platforms and publishers is that the goal of creating such a distinction in the first place is to restrict the ability of the actor to undertake certain policies.
A billboard is a publisher; the content there is clearly governed.
So the criteria seems simple:
* is content freely posted? What is the approval process for content?
* if content is freely posted, are the rules for removing content uniformly applied and viewpoint neutral?
* what mechanisms exist for appeal, protest, modifying the rules, etc?
Platforms are defined by what they don't allow.
Publishers are defined by what they do allow.