beige/brown/khaki
polo shirts
flare/bootcut/pleated pants
shorts (except for running, around the house and rare super-hot days)
more than one non-basic color at a time (If I'm wearing red or orange it'll only be with grey or black or blue or something else that's very plain)
layering (I get too hot. My only exceptions are with a vest + ONE sweater that needs a white shirt underneath)
accessories (I carry a bag and will wear gloves/scarf/hats if it's cold, but I hate watches, pins, jewelery and all of that sort of thing)
voluminous pants (Maybe I'd try something really unique, but for the most part I like my pants slim, but not necessarily skinny)
obvious kitsch/camp/irony
blazers with padded shoulders (unfortunately I have a few that my father 'helped' to pick before I knew enough to care, but I wear them as infrequently as possible)
Anything with a lot of pointless "stuff". It all has to make sense to me or seem purposeful. I'll buy some pieces with prints or color or pockets, but it can't just be something randomly attached or I will disregard it.
Last but not least, I don't like to wear anything that doesn't speak to me on some level. I know it may sound overly particular or snooty, but I'm not in the habit of buying "random" stuff (at least not anymore), so anything I'll wear has to have some point of interest. It could be some little detail, the fabric, the color, the cut or whatever. It just can't be boring or average (even if it is simple/plain).