It isn't really compatible with HTML5's input "required" attribute. If an input has the required tag, and you press the submit buton, and the field is empty the browser will fire the "Please fill out this field" message, BUT, you also just disabled that submit button. So in effect, the form can no longer be submitted.

The redirect-after-post pattern is nice, but it doesn't solve this problem. The problem is either stupid users who double-click the submit button or impatient users who re-submit the form before the page has loaded. Both of these cases take place before the redirect-after-post pattern applies.