Grey out the "Post comment" button until you've actually typed something in the box
I know the button is inactive, and I see that you are actually changing it subtly, but I think not enough.
I was just doing stuff in multiple browsers, came back and saw the big blue button and thought "oh, I didn't actually post that", clicked it, and then worked out what was going on.
Either make it a very faded blue, or grey, perhaps:

Right. Button styles are from Bootstrap, I haven't changed them. I've been meaning to get to it, though.
How about now? (Also, please let me know if you hate or even slightly dislike the new button style. I spent too much time styling them and trying to get away from bootstrap defaults, but I still feel like something's wrong.)