There's a great blog I linked to once before that posts about interesting book covers. Today's post compares the US and UK covers for a book called "The Bush Tragedy." The difference in tone is pretty astounding. Personally, I like the UK cover a lot better, but can totally see why a US publisher would go for the safer and more serious choice.
It's one of the most interesting blogs I read, but I wish they went back and did follow-up posts about sales figures and successes vs. flops, to see if the cover really seems to make much of a difference, or if the writer's impressions end up right or wrong as far as the ultimate success of the book. Of course, factoring out the effect of a cover, when there's marketing budgets and exposure and subject matter and author fanbase and reviews and the quality of the book itself, is probably impossible.
Although if I were a publishing executive, I might be tempted to release a book like this with two very different covers, randomize which stores get which, and see if there's measurable differences in sales... and then perhaps swap out the less effective cover for the other one and see what happens. Frankly, there's probably twenty-five ideas just like that, with everything from cover art to author photo to marketing messaging to the blurbs on the back to the flap summary to the font size that, given the number of books published every year, and the number of roughly-identical bookstores each one gets displayed in, that I think publishers perhaps ought to be experimenting with, just to get some better sense than they have of what motivates book sales and whether there are in fact some easily-manipulable things they can do to get people to make the decision to buy.
At the very least, the first publisher to do something like this would probably get some media attention for it, and that alone would push some sales.