Absolutely nothing, LOL.
Everyone's definition of casual is different. I find all NATO straps casual. Leather NATO is just NATO pretending to dress up. If you want casual NATO is always casual. If you don't want casual, I would avoid NATO. Also, not a fan of the seagull with NATO strap above. The arabic font, circular/rounded bevel and knurled oval crown are just too dressy for the strap.
This is all my humble opinion of course. I wouldn't think anything of it if we met in person.
My thoughts exactly, which was the reason for the question.