No good discussion/update flow, groups aren't really dynamic enough, and getting a google account isn't going to appeal to the kind of people who dislike facebook.

(FWIW Meetup does completely replicate this use case, but they charge money)

exactly. Even with meetup, its not perfect for 'I want this specific group to come over.'

In a lot of ways, mid-2000s evite would be perfect -- but there's the spam / nagging side of their business that spoils it.

To be fair, I have no idea how one would monetize my dream events platform.

is monitization terribly necessary? The server costs of an open-source type of meetup should be negligible. Hypothetically a lot can even be done with plain email, provided users set up a filter to send all the invites into a separate folder if don't want to get distracted.

somebody always pays. Ideally it would have a strong app and desktop version with a standard calendar export... not too far off from facebook today, just omitting their other garbage. It wouldn't be too server intensive, but if people are using it, there's a cost.

Also, keep in mind that not everybody is as savvy as the people who typically participate on HN. It should be as simple and minimal as possible.