I'd love to know more about what you liked about Google+ communities, what worked, what didn't.
I'll admit, I did love the private aspect of Communities on G+. I have several (golly, many!) learning groups on G+. I've been at the helm of the "G+ Mentorship Program for Photographers" for years now getting folks involved in free photography mentorships and run the "Mentorship for Mentors" to teach folks how to mentor. Those kinds of things are better when the public isn't looking over the shoulders of those trying to learn ... as well as mentors who are volunteering their time, giving it a go to pass on their knowledge––it's nervous-making enough as it is when it's a mentors first time with a group of 20-30 people ... ;oD. Plus, I'm one of the photography Masters at Trey's Arcanum and we've been running our communities there ... also private. So, yes, I do love the wonderful things here on Cake, but ultimately hope for a place with Communities/Groups that can be private too. And display big, beautiful images, of course ... ;oD.
I can't think of what didn't work with the Communities there. I've had grumbles/annoyances, of course (missing posts and the like), but for the most part, they work well.