I don't think the answer is so binary. I think non-rounded corners convey a feeling of simplicity. And because of that, I believe using rounded corners in a simple design can actually be harder on the eyes. Take HN for instance. If the corners were rounded, would it really be easier on the eyes? Well, I guess we would need new users to verify that.

Or take Microsoft's or anyone's tile UI for instance. I think the tile UI gives a feel of simplicity. If the tiles were rounded, would it really be easier on the eye?