A book's cover will often be enough for me to pull it off the shelf and give it a look, but I wouldn't buy it on the basis of a cover alone. I always read the first few pages, a few pages in the middle, and, of course, taken into account any other work I've read by the same author.
There's a language in book covers that signals clearly to people what the book is like. Publishers and designers almost always conform to the same set of rules.
Cop-type mysteries: sans serif font, author's name large and visible. Dark, masculine colors, some shine but not too much, hard graphic lines.
'feminine' literature: scripty fonts, author's name less visible and small, muted colors and images.
chick-lit: some brightish neon colors, playful fonts, sometimes different colors in the lettering, weird trim sizes.
gift books: small 'special' trim sizes, expensive print job, elegant appearance.
trendy cross-gender-appeal lit: san serif fonts, straightforward colors, minimal graphics, usually punching up the author's last name a bit for recognition.
as Halo says, I could go on . . .