OMG! This, so much. Just having your own space to vent and play. Your rules, no worries about censorship, and if people wanna come play on your playground, well, you're da boss! (Don't mind me, I'm writing wiki pages on my site...)
This is my favorite thing about it. Owning my own platform and not having to worry about it disappearing. I control everything, and I'm the one that's going to decide the future of it, not someone else that can easily pull the plug when they feel like it (think MySpace, etc).
That's the main thing that's great about owning your own community. You're the backbone and the spirit of it. It's yours. It's your life and blood.
The playground aspect is also one of the greatest things too, I can learn new things, test out various ideas, and go from there.