Ok, that's not quite what Tom B. says, but it's pretty close:
So let's talk about covers a little bit.
It used to be, back when comic books were sold exclusively on the newsstand, that the cover was what sold the magazine. That's why so much attention and effort was spent on the cover image. There was no advertising, no promotion, no Wizard magazine or Previews catalogue or Newsarama to let people know what was coming out months in advance. In fact, other than if it might have been mentioned on the Bullpen Bulletins page, or shown in a house ad, you didn't have any idea that a new book was coming until it arrived. So the cover was a crucial component in making your title sell.
We still put crazy amounts of effort into our covers--but one of the unspoken truths of the industry at this point is that the cover is no longer responsible for selling the magazine, and may in fact have a negligible effect on the total sales. Sure, a really good cover may be able to hook a few extra people into picking up the book off the racks, assuming it's there for them to find, but the whole mechanism of our distribution and retail system makes the cover close to superfluous. Retailers order their books months in advance from the Previews catalogue, as do a great deal of the customer base through pull lists. And once you get outside the big coastal cities especially, the amount of display space a given shop has is relatively miniscule. I don't know what proportion of the average shop's books are sold off the rack as opposed to through pull list subscriptions and advance orders, but I'd hazard a conservative guess that it's probably half.
There was a time not so long ago when it was Marvel policy that every cover should have a single iconic figure, and no direct relevance to the story in that given issue. And fans far and wide screamed about it--yet it did nothing to hurt sales overall. And now that we've returned to mixing up the cover approach, it hasn't materially affected sales either. I regularly hear from a small group of people who don't like the mostly-iconic covers we've been running on NEW AVENGERS, but those covers clearly have not been hurting the sales on that book--and the more story-driven covers on, say, THING didn't materially increase the sales on that book.
The place where cover art can help or hurt your book's sales these days is really the Previews catalogue, the tool that retailers and readers use to advance order their books. So it's not like the cover art is irrelevant or anything. But even within that venue, just having a strong image isn't enough in most cases to sell the magazine--other factors such as story content, creative team, relevance to the larger Marvel Universe, and the amount of coverage given seem to be more important elements in making a decision for most retailers and fans. I'd hazard a guess that, were we to solicit a new ULTIMATE project with, let's say, Brian Bendis writing and Greg Land penciling, but we didn't show any image at all, it would still be ordered quite well--retailers would be very upset with us, because they hate having to take a position on any title without the maximum amount of information they can get, but I expect that most of them would weigh the factors they did have--especially if the story content promised important elements for the ULTIMATE line--and would order acordingly.
So, does the cover matter anymore? Well, everything matters. It's just a question of degree. And at this point, the cover no longer has to shoulder the promotional weight that it once did--so it matters perhaps a little less than it once did.
Covers have always upset people. Most of them depicted things that never actually happened in the comic. Often they were drawn by a better artist than the one responsible for the interior, which is misleading. Many had word balloons, which Wayne says is an absolute no-no. And lately they've been doing what Tom calls the 'iconic' covers - covers that are just generic pictures of characters that have nothing to do with the plot of the comic. Obviously even the iconic covers take a lot of work - it's just that they can stock them up in advance and use them as needed. I like the iconic covers the least. Every once in a while is ok, but 12 months in a row of "Punisher stands in front of a wall" is too much, and those early New Avengers comics featured characters that weren't even on the team yet (and they weren't drawn by the interior artist, so that matches 3 of the above negatives). But if the thinking is that the cover doesn't affect the sale anymore, then i don't see them changing their policies any. Although for what it is worth, the house ad for Annihilation, showing the four (iconic) covers of the four miniseries, was probably the main reason i decided to get that event.