I think that a your crop is exactly in the wrong spot. If you would have included the cut off corners or even cut all the corners out completely it would have scored better. Also, if you're white was a bit brighter it would help.
Yep, that's a straight edge! But as a standalone photograph, it's not particularly interesting. If you wanted to use this for stock, you'll need to have your ruler even sharper than you have it here (though it's not too bad) and you should probably include both corners.
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