This and the button style (which to me don't look enough like buttons to be 100% clearly identifiable) would be my criticisms as well. Other than that it looks nice.
I just looked at the demo again and it is only the <input type=… buttons that (since they have no extra styling compared to the other inputs?) don't stand out enough, when I imagine the <button type=… ones next to a form they look fine. (I personally like a 3D-effect on buttons, but that's really a matter of taste and doesn't always fit the other elements)
I think it's due to people not having really bright screens. If your screen is too dim, you need the background to be as white as possible. Another factor is that off-white can be percieved as dirty.
Ideally, I'd prefer if off-white was the standard for backgrounds, and you could use bright white for highlights and more contrast in images. Or maybe 100% white should be the standard, and you have a screen that goes to 110% :-). Maybe one day when HDR screens become standard...
I know. I was speculating why some website creators prefer dark grey on bright white, as opposed to pitch black on slightly off-white (which I think is easier on the eyes).
Line ghosting [0]. Which can be alleviated with more spacing between lines and a larger font size, but even then I still find it problematic personally. "Ghosting" is a lot less noticeable on light backgrounds, which I think is just how the eyes "work".