I really don't think that a pop-up window and a form have much in common from a UX perspective. The fact is that pop-ups interrupt you with a nag, and the user

I really don't think that a pop-up window and a form have much in common from a UX perspective. The fact is that pop-ups interrupt you with a nag, and the user's desire is to get rid of it as soon as possible (regardless of which action they actually want to take). Putting the text on the button itself speeds this process up, and reduces errors. Submitting a form is a MUCH MUCH slower, and more thoughtful process. You can make a snark about replying quickly to a comment, but have you ever WATCHED a user deal with a message popup they've seen before? Blink and you'll miss the whole thing. Even more importantly, most forms have only ONE button. I bet you could label it simply "Button" and a ton of your users would never even notice.