I usually notice fashion faux-pas that guys make because I practically dress every guy in my life by buying them clothes that are in style as gifts and sometimes randomly.
I've noticed that guys generally think that sloppy is cool, like if they don't iron their clothes then they seem laid back and hip, but girls don't like that. At least any girl that is girlfriend material. Girls want a guy who is clean and organized, because it reflects their life and personality.
Another mistake guys make is wearing socks with sandals, shorts with Jordans, or wearing outrageously bright colors.
But these are just my opinions and the things I see when on my college campus.