So what happened to Episciences? The Episciences project was all the rage a few years ago, back in the days of the now-defunct math2.0 forum and the Cost of Knowledge boycott (ca. 2013). Its goal is to build "epijournals", which are journals trimmed down to the one essential ingredient of publishing -- peer review (as everything else, such as archival, dissemination, to some extent also formatting, is handled just as good if not better by repositories such as arXiv and HAL). Of course, this reduction of editorial (and other) overhead takes the wind out of the sails of the "who is going to pay for it" argument.

The project was supposed to be launched shortly, which I understood to mean in a matter of months. Fast forward to April 2015, there is only one epijournal running. What has happened? What type of difficulties came up? Not enough universities lending authority to the fledgling epijournals? Disagreements on the way forward? Political sabotage? Obsoletion in favor of a newer, better project? Or is it just moving on, slowly and steadily, and what is needed is just more patience? Any answers would be of interest, as other open-content initiatives might face similar problems.

EDIT: Andrew Stacey has resurrected the math2.0 archives, so I can link to the old discussion of the episciences delay there. Most relevant is the reply by Benoit Kloeckner (Mar 20th 2014):


  About the lack of communication about épisciences: I understand your frustration, but since the technical team is small the development has taken some time, and I feel that keeping communicating without having much new to propose could have been counterproductive. People know it has been proposed, and communicating largely once it is up and accepting submission might be more effective.
  
  When we launch, there should be a couple of starting titles as well as a standard way to process applications for creating new epijournal. Basically, an epicommittee (which has been gathered already long ago) will be in charge of examining the applicant titles. If you feel that a new journal is needed, or that a journal you are participating in should join, you can start gathering a editorial board or discussing the question in the journal and with us (contact@episciences.org).
  
  Finally, I prefer not give a tentative opening date as the previous ones had to be delayed. Things are moving forward, please keep the project in mind but be patient.


EDIT2: The journals page has grown! There is now a Hardy-Ramanujan journal (the link on the Episciences page has typos, whence I'm linking to it here) and DMTCS (which, IIRC, has been something like an epijournal since before the episciences project). It looks like more is coming up...
Traditional, subscription-based publishing, with self-archival of pre-prints, works well. 

That is probably the major reason why other models stay confidential, or are plagued with dishonest behavior (like author-pay, "gold" OA).

The other aspect is that such initiatives are typically pet projects driven by a few enthusiastic individuals. Sometimes their institution tells them to do what they are actually paid for. And when they leave their institution, stop their academic career for a reason or another, or retire, there is no real incentive to find someone to take over.



Side note:


  Of course, this reduction of editorial (and other) overhead takes the
  wind out of the sails of the "who is going to pay for it" argument.


Not really. In the "green" OA model, somebody is obviously paying for archival. It might be invisible to most authors, but institutions have to decide to allocate a portion of their budget to maintain these databases. As for formatting, it's payed via the salary of researchers who spend time on such a task instead of on research and teaching. Highly educated scientists having to worry about font size, picture format, placement of floating objects and 'overfull hboxes'... The cost of knowledge, indeed.