• One thing that bothers me hugely is wearing clothes that are in a size too small. And I don't mean "form fitting" or necessarily "tight pants" (because tight pants can look good, hello skinny jeans) -- but it's obvious when clothes are meant to be form fitting opposed to clothes that are simply the wrong size. It's never flattering, and looks awful. If you're slender you look strangely disproportionately little, and if you're voluptuous you not only look heavier, but you have a nasty little muffin top.
• More than one denim piece worn at a time
• Wearing a single designer from head to toe, e.g. Chanel skirt, Chanel blouse, Chanel shoes, Chanel bag, Chanel stockings, and Chanel earrings for one single outfit. It's very passé.
• Visible nipples and "camel-toe" seen through clothing. I know sometimes we can't help the former, especially in times when the temperature drops, but a bit of modesty doesn't hurt anybody.
• "Matchy Match" -- wearing a single colour or pattern (especially pattern) from head to toe. I've noticed the only exception is black, but even then the outfit is incredibly boring without the proper mix of texture and material.
• Wearing clothes out of the season, e.g. snow boots in the summer/flip flops in winter.
• Shirts with "sweetheart", "naughty", "baby", "princess", "hottie", etc. Why? Just why? Especially on young girls who wear the "hottie rescue" and "sexy".
This is all I can think of at the moment, although I'm sure there are way more worse things out there. I tried to exclude things that are more personal style, e.g. graphic tees and leggings -- which although may be things that I don't like -- are certainly things I wouldn't go as far as to say are "wrong" in fashion.