An "us against the world" founding cohort bent on seeing (or making) enemies where none existed, a fundamentally toxic anonymity dynamic, no effective user-focused moderation tools, staff incompetence, indifference, or blindness, AWOL forum moderators, brigading, and more.
The cohort problem is similar to other disaffected diasporas, across the political spectrum -- Voat and Imzy couldn't be more different in outlook, but the behaviour was quite similar. This is why founding cohort is so crucial. Usenet, Slashdot, Reddit, Facebook, and HN draw from a similar (if highly nonrepresentative) group in that regard: larrhely techy, college-educated, wealthier, male, and generally culturally privileeged. There are of course numerous individual exceptions, but this is the general trend. Finding a formula for a more inclusive, but healthy, founding group would be a Good Thing.
I count among G+'s negative foundations the strong tepresentation by marketing and SEO types early on. Now the challenge is (frequently Asian) sex and pr0n bots.
Edit: this looks like a project with phenomenal potential that I would definitely be interested in using.