I'm excited to see how multiplayer is implemented. When I first heard it was happening, I thought it would be like a small server of <6 friends just messing around at the KSC or other colonies. Sounds fun for sure, but mostly contained and not granting full access to the entire play area of the game. Then I heard them talking about players trading resources on colonies, and it broadened my horizons.
I don't agree that it will be an MMO, as I can't imagine that fitting KSP. Instead, I imagine an experience like Minecraft, where a chill group of 5-20ish people log on for an hour and make bases and cool projects, sometimes together and sometimes on their own. For example, you could try to have a massive community refueling station. Or have challenges, like the first person to fly into the sun or redirect an asteroid to hit the KSC. Servers could have rules like not leaving garbage all over the system, but that also brings in the idea of kicking/banning people and admins for servers.
KSP has always been a single player experience for me and I ultimately don't need it, but it has a lot of potential for this community. For instance, I love watching the RSS/RO/RP-1 community and if some of the more prominent members got together and recorded a playthrough, people would love it. This is all just speculation, though.