This afternoon - while we were testing out several forms - my team came across this issue where a required checkbox didn't display the "required" message when the field was left blank:
Instead, a red box is shown around the field. I suspect this is a bug - that hopefully Marketo is aware of? Just now, I saw that Sanford Whiteman posted a blog post that provides a temporary workaround: http://blog.teknkl.com/forms2-bug-checkbox-error-msg/
Reprinted it on my MUG blog too: New(ish) Marketo Forms bug w/required Checkbox fields, and a fix but I didn't push it to Products.
I'm hesitant to flag smaller Forms 2.0 bugs as fixable (as opposed to workaroundable) because another regression could be worse than the initial bite...