Is it the bad cover with the man titty or the half naked people on front? Is it a terrible title? Did they read the first page and decide it was dreck? Was it a poorly written back cover blurb? Or, as simple as the genre not being a fit? Maybe the title made you think contemporary but the book is historical.
So, when you're browsing for books or maybe digging through your To Be Read (TBR) pile, what makes you throw down one book and move on to the next one?
(For me, a bad cover will make me drop a book like hot cakes and a good cover will make me buy a book without reading the blurb)