I can't believe I haven't done my "soapbox rant" on the subject of covers on this board. Pooh--I'm not sure what "pants" implies--but I'll assume it means something like "whittle".
First, let me say that I am not a fan of Paul Kidby, and when I was a bookseller, I'd have been even less of a fan. The point of a cover (dust jacket) for a hardcover is to catch the customer's eye. And Kidby's covers only do that in a negative way. I know, I know--everybody (except me) gets a big kick out of figuring out who's who, or what picture, statue he's parodying. But that assumes that the reader is a) familiar with all of Pratchett's works, and b)doesn't mind busy, cartoon-like covers. Now I think that the British cover of Thud! (which is substantially different from his earlier ones) is brilliant -- artistically and from a bookseller's point of view. It's gorgeous and draws the reader's attention into what might be going on in this book. The HC one isn't particularly good--although it does pick up the troll element and has the virtue of being fairly simple. I've never seen the American HC of Thief of Time (and can't find a picture of it). But I think the cover of the paperback from HC is far superior to either of those you showed Pooh. I'll try (or Tony will) to post a cover as an example.
And then I will say that the whole American publishing world has become more and more profit driven. And that means they hire people right out of art school to draw their covers. Obviously, some of them have never read the book and/or know nothing about the country where it is set. One of my favorite examples of that is the J.A. Jance Partners in Crime. This is the first one where she partners her Seattle and Cochise County, AZ detectives. The cover is quite striking-- dove gray with copper colored lettering and a gorgeous copper colored photo -- but it's of Monument Valley which, though it is in Arizona, has absolutely nothing to do with the book, the characters, or the author.