If you setup your own discourse server you will have the option of connecting to the main discourse site or not. There will be some benefits to connecting, but you are not required to do so. Therefore you will not have to share a user base with the main site.
The main core of discourse is currently GPL and the main developers push and pull via github, so it’s very open and visible. The is the possibility that they might start keeping a proprietary version of the code and develop on that, but if they do it will again be obvious, and the code they’ve already released will still be available via GPL. Just fork it and you can use it forever without fees. I doubt they will do this, especially in the short term. What they need right now is to develop the software while under GPL so it gets a lot of traction in the marketplace.
The software is going to be the same for everyone. Whether you connect to the mothership is a configuration option, nothing more or less. If you connect to the mothership some additional features come into play, but that’s because those features are supplied by the mothership, and they are primarily features related to username validation (ie, I will be @stienman on all discourse connected sites and no one can take my username, unless they sign up to a non mothership connected site) and also the “global community” feature, where all the forums I’m participating in are listed on my discourse.org/users/stienman page. There may be additional features later, but this is open source. If you don’t like it, you can make your own mothership, and create a bunch of forums that connect to and validate from it.
Regarding the license, they intend to sell non-GPL versions to users that will NOT run GPL software in their enterprise. So, say ESPN comes along, likes the forum, but their legal department won’t allow them to use the GPL open source version because they are afraid they’d have to release their proprietary internal API or code once they adopt this software. They can go to discourse, buy a version of the code that is under an enterprise license, and they get exactly the same code you get under the GPL, but they aren’t required to follow the GPL.
This licensing method is used successfully for Mozilla and many other open source projects. They are thus both open source and commercial friendly.
Some people are worried about contributing code to the project, because they have to give all rights, not just GPL, to discourse. Then discourse will make money off their contribution if they sell enterprise licenses.
If that’s a concern you have, then don’t contribute to discourse. Fork it, and make your changes, and release your changes under the GPL. They can’t take your GPL code and sell it as non-GPL code, so it’s as open source as you want it to be.
However, I expect a lot of people will still contribute to discourse just like people contribute to Mozilla and many other open source projects because their code will actually help the company to grow, which means better software in the long run, and higher adoption rates, and their code will still be available under the GPL.
But the code is GPL, it’s going to remain released as GPL for those that want it, and it really shouldn’t be an issue except for GPL purists who really don’t like the idea that someone is using the GPL as part of a dual license system. They also don’t like Mozilla for the same reasons, but guess how many people around the world use Firefox for all their browsing?