Following on from Adrian's question, is there anything on a cover that will make you pick it up or walk on by? For me it's pastels. I may like to read romantic fiction but doesn't mean I'm going to let the publisher make fun of me for it! My reading list is cut most frequently by unacceptably pink and frilly covers. (Not literally frilly, but you know what I mean. Cartoon ribbons and bows are as bad as the real thing, if you ask me.)
On the other hand, you also won't catch me picking any of those grey books. You know the ones I mean - 20 shades of 'let's all get on this bandwagon'.
What, on a cover, will make you tut and look the other way?
I have to say I don't think it's picky or sad to expect a cover illustration to be from the correct century. Ten years either way is forgivable with something from the distant past, but one hundred? No, that cover needed changing.
Because I read a fair amount of historical fiction it's fairly common to spot something not quite in period. Maybe I'm just sad ;)
For example, there's a series based on the War of the Spanish Succession (1701-1714) where the first book cover illustrated a Napoleonic period (1813) cavalryman. In this case I picked it up based on the cover and was disappointed by the blurb. However, I still bought it. More fool me.
The publisher subsequently changed the cover.