The weight of the names in this thread is really something. I'm constantly amazed at the quality of what is posted on Mathoverflow. Kudos to them for harbouring such a community.
Something intersting about MathOverflow is that it started as an independent site, with the creators eventually agreeing to make it part of the StackExchange network. I believe part of the agreement allows the owners of MathOverflow to break off from the main site again, in the event of a major disagreement between MO and SE. I'm sure they're glad to have an escape hatch given the recent aquisition!
Heh, I guess that makes them the Texas of StackExchange.
Somewhat related: I just learned of an Open Source rival to StackExcange by the name of Codidact. [0] Looks like they have an active math subdomain. [1]