Thousands of user registrations advice Bit of a generalist question but in case someone has anything to share... I am thinking of building a campaign site in Craft that will register 15,000+ users. Is there anything I should be aware of or pay particular attention to?

The users will have a front end registration and a dozen custom fields.
I would also think about spam bot registrations, which can be a big problem, and the more popular your site, the more attempts you're likely to see.

I'd add something like Google recaptcha to avoid that...we use this: https://github.com/aberkie/craft-recaptcha - and have had zero spam registrations since launch.  

With our old site we used to get them all the time (not Craft but a form is a form, so that doesn't really matter I think!)