Clear and to the point. MailChimp gets out of the way on this on an email that needs to be quick and clear.
I wonder if “Just one more step…” is the right heading on an email that really needs me to activate my account. These kinds of choices can sometimes be brand driven or tested. The most important thing is to make sure the customer is clear about what to do.
Note: The “username” title here was my username and I replaced it with default text for my own security