Have you read Seam doc on this subject ?
Chapter 9. JSF form validation in Seam
..Note: specifying @NotNull on the model does not eliminate the requirement for required="true" to appear on the control! This is due to a limitation of the JSF validation architecture..
If you leave the field blank JSF will not consider it a value change therefore the s:validate tag logic will never get triggered, thats why you get a Seam debug page instead of a nice formatted validation error message.