The steemit team released an update about upcoming communities and Hivemind here.
From the post:
Hivemind (or “hive”) will power Communities on Steem, but that’s only one important use case. It’s designed to be a developer-friendly layer on top of the Steem blockchain that will enable any developer to access a database containing all the social information stored on the Steem blockchain without having to rely on minimalistic core blockchain APIs.
This is my personal view of the blog post. It's pretty technical.
From how I understood it, is that not all the information in the blockchain is consensus critical.
Which basically means it does not have to be verified at all times.
Who you follow for instance or your reputation score is not consensus critical.
What is consensus critical is the amount of STEEM/STEEMPOWER and SBD that you have. This needs to be known at all times. (You don't want people to transfer STEEM they don't have for instance).
They want to separate the consensus critical information from the non-consensus critical information.
why?
From the post:
Much of the information that app developers want to leverage is not consensus-critical; that is, it either does not belong in a blockchain node or it doesn’t need to be accessed directly from one. Instead, we can store all the relevant information in a traditional database and keep it in sync with the blockchain, while adding new custom layers of logic and features. This is exactly what hive does; and because it’s written in Python, developers can modify its behavior and APIs with ease... and experiment to their heart’s content.
Pretty cool. It will make it easier for developers to play with the non-consensus critical information without having to go through a full node.
Hivemind will be implemented in stages.
The first stage won't affect any of us if it all goes well. It's a back-end upgrade. But it's very tricky
from the post
It’s akin to upgrading an airplane engine mid-flight while making sure there are no hiccups and all controls work as usual.
Serious stuff.
If all that goes well the fun stuff starts. Communities being one of them!
from the post:
Communities bring a whole new form of interaction to the Steem blockchain, and we are excited to see the ecosystem evolve once users can collaborate easily with like-minded individuals, curate topics of shared interest, and truly organize. We anticipate that the Steem blockchain will become the place to be to discuss everything and anything under the sun.
I talk about it more in my vlog.
I am a witness with our developer group 
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