I having this issue as well. In my case when I render the registration page and look at it through firebug, I see that all the javascript related to field validation is commented out. However, the post, or before post validation works as expected.
Problem found: In my case the template I was using was loading a jquery.js script after the community builder's scripts, which included jquery as well. This caused problems when the ready document event got called.
If you run into this problem check to make sure that your template, or your pages, are only loading shared javascript libraries first and upfront.