On the old MO site, we had an extensive FAQ describing the purpose of MO and the kinds of questions that are welcome. Right now, I don't see any way for a new user to figure out what we're about and how we differ from math.stackexchange.com. I see this leading to lots of unnecessarily hurt feelings. How shall we deal with it?
There's an extensive help system built into the site, with a directory of pages here: https://mathoverflow.net/help
By default, moderators here can edit this page: https://mathoverflow.net/help/on-topic and certain sections of this one: https://mathoverflow.net/about (including the example question shown, although at the moment it hasn't quite figured out that there are questions here yet).
If additional pages or customizations are needed, don't hesitate to ask - it's not always possible, but we'll see what we can do.