Your form lacks affordance - it's not clear, despite the subscribe header, that one should complete it by tabbing/clicking and then entering data. On the OP ex

Your form lacks affordance - it's not clear, despite the subscribe header, that one should complete it by tabbing/clicking and then entering data. On the OP example at Huffduffer one clicks a "sign up" button (which has the mouseover feedback you lack incidentally) to get to a page which is for sign up. Hence the affordance is not so important as they're already in the "I need to enter data"-pipeline. Also FWIW I think messing with default button formats should be avoided unless you're going to make sure that it's clearly a button (affordance again).