Rather than the old, tired concept of communities as forums, Sensio Connect is a tool to build your own systems on top of. It sports OAuth2 authentication and so integration is dead simple.
Components and platforms
I think it goes very well with the Symfony2 model. Instead of building one big, monolithic system you have a set of decoupled components. When used together they are more than thair sum.
This is especially interesting now that I am in the process of starting up a Swedish Symfony community. We will see how we can leverage this platform.
If I am to mention one slightly negative thing it is the fact that it is not called Symfony Connect. Lately we have seen projects like Silex and Twig move to the sensiolabs.org domain and this all feels very calculated to me.
Of course Sensio want to increase their visibility - it consequently increases their business and the money they make. And I really think they are worth every cent and more so.
Though it still leaves a sour taste in my mouth. But maybe I am too idealistic when it comes to open source. Or maybe it comes from my bad experience with mixing in too much business, from other open source projects.
All in all
To sum it up I think this is a very exciting project and I will watch its outcome closely. I am certain we will see some interesting projects spring up around this. Like we have already seen Symfony2 Bundles being enabled by the GitHub platform.
I have never heard of another open source community having done something similar, in which case this is extra awesome. But maybe I have just missed it?