As a lefty, my biggest problem with whiteboard interviews is writing code on the whiteboard. I just can't write text in straight lines very easily because my hand is occluding it, and so I write slower and use more strength doing it, getting tired pretty quickly. I have no problem drawing on a white board (I draw my shapes in counter directions like most lefties), or writing simple labels/lists, or whatever, but coding on a freaking whiteboard should be illegal :).

And then they expect me to talk while writing code on the whiteboard...