I agree on the great lines. However, most of these are in very remote areas, which is why I would include the the 45 degree latitude line (I even have a picture of that one.)
I would not include border crossings, since there is already a category for those. However, to have some longitude lines too, I suggest including the borders of time zones - the are ideally at 7.5 deg., 22.5 deg. 37.5 deg. and so on every 15 degrees east and west, but are adjusted to country and state borders.
This definitely has potential...