The way I see it, the cover of a book says a lot about the book itself. We're not supposed to judge, I know, but at the same time we have to realize that people are going to judge a book by its cover. That's why it's such a problem to sell a book in the stores (like Borders). Most books end up spine out rather than cover out, and as such are less likely to get snatched up and examined by the ravenous book consumer.
One of the things that I do when I'm in the store and walk around the new fiction area and look at the covers. I pay attention to the image, the author, the title, and anything else on the cover that might be of importance. If I don't recognize the name, the cover is what has to sell me enough to pick up the book. There are too many books in the store for me to pick up every single one and give the same amount of attention to (by reading the back, for example). The job of the publisher is to attract my attention.
Likewise, the publishers need to be aware of their intended audience. Luckily Stross' new book will likely sell just fine with either cover, since his name is well known anyway, but if Stross were a new author with a cover like that, I can see it having issues. Stross' audience are hard SF readers (and near future readers). As such, the publisher has to market to that audience, unless this new book is a significant change from his usual, in which case they have to figure out what works for that audience. If you have an epic fantasy book, you need a cover that fits it. Don't put flying bunnies on it unless it's a book about flying bunnies (and probably humorous). A science fiction book needs something that not only tells us about what's inside the book, but also targets us, the scifi reader, with imagery that screams "scifi". Computer animated figures don't scream "scifi". That screams "cheap". Many people don't realize this (or maybe they do), but there are entire website devoted to selling pre-drawn figures like the one on the cover above. All of them are obvious and while I respect the talent it takes to be able to get to that level of artistic talent, the art itself is not "great stuff". It's amateur. Video games produce better looking characters these days.
The quickest way to get me to not touch your book is have a horrible cover. And the quickest way to get me to think your cover is horrible is to have something that looks like it was drawn on a computer. The trick about computer art is that it has to mask the fact that it is from a computer (unless, for some reason, you need a cover that looks like it came from a computer). Yes, you can do great things with a computer, but it takes practice and talent. It looks amateur to put obviously computer-based art on a book cover when you could just as easily find a piece of hand-drawn art for the same price. Even crappy hand-drawn art looks better than the computer drawn figure. Yes, I'm saying it. I've seen my fair share of poorly drawn book covers, done with real art, by hand. Ten times out of ten, it's better than any of those annoying computer figurines you see on some covers. A good bit of advice for publishers, particularly the small presses (and I'm not pointing out anyone in particular here, but this is mostly aimed at those of you who do a lot of e-books), is this: it's better to take a piece of handdrawn (painted, etc.) art that doesn't look all that great than to take a silky and shiny computer-based drawing of a figure that looks just like every other computer-based figure that has been drawn. Seriously.