Flip flops have to be #1. Especially if you're a dude.
Pretty much anything with overt branding. This is a catch all category for anything from Polo and Nautica to Ed Hardy and Affliction.
Light or "stone" khakis, especially when they're pleated, baggy, and paired with black shoes.
Fedoras. The Indiana Jones/Dungeons and Dragons dork variety, and the hipster/1980's variety are equally heinous.
In the interest of full disclosure there are also those things that drive me nuts these days that I've personally been guilty of wearing in the past: ribbed sweaters, baggy/raver jeans, gangsta boots, and wrap around shades.