I have been a Dark Sun fan since the 2E era. It was an interesting departure from the sword and sorcery fantasy, more John Carter of Mars than Lord of the Rings. It really let me rethink how to play the game since the things I was familiar with no longer behaved the same way.
For the most part I think the 4E version is pretty good and I've really enjoyed playing the D&D Encounters running it. It maintains a lot of the harshness and brutality of the original but I think still manages to stay pretty balanced. I liked that they use character themes instead of new classes to represent the Dark Sun specific classes like Gladiator and Templar. I've seen people talking about power creep with themes but Dark Sun used to recommend (if not require) players start with level 3 characters. Themes help bring first level characters up to a point where they can reasonably survive encounters. In terms of gameplay I'm really happy with the Dark Sun books.
Now to my gripes since they're a bit more specific than my praises and have more to do with the presentation than the gameplay elements.
I completely agree with everyone else about the generic artwork. While the original Dark Sun was defined in part by Brom's artwork I don't expect WotC to just dust off old pieces and reprint them. Nor do I expect them to tell their current artists to simply copy Brom's work. That being said I find the current artwork to be not only uninspiring but a bad fit for the setting. As others have mentioned much of the art doesn't correspond to the copy or even the general description of the setting. Folks are running around in generic fight scenes bedecked in metal gear. Even the black and white line art in the original books did a better job setting the scene than the fully color photographic art in the new book.
Besides the art just being bad it also doesn't help you visualize the world. If you look at Brom's cover artwork for the Prism Pentad novels not only do they show some crazy characters but some really detailed background scenes. A cursory look at those covers alone help you get a good visualization of the Dark Sun setting. Even the few bits of art in Dungeon and Dragon magazines for their 3E Dark Sun conversion are more effective than most of the art in the new book. I hate to harp so much on the artwork but it's the book(s) major downside. I don't think players completely new to the Dark Sun setting will have as good of a handle on the game world as people who are familiar with the 2E version.
Besides the artwork I am really disappointed WotC didn't do three Dark Sun books. I think they really good have used a Campaign Setting, a Player's Guide, and the Creature Catalogue. I think they did a pretty good job of squeezing roughly ten splat books worth of setting materials from 2E into the hundred or so pages they had available in the Campaign Setting. Had they put the new races, powers, and themes into a Player's Guide they would have had a lot more room in the Campaign Setting for more artwork to match the copy even if it wasn't as good as what was in the old books. They wouldh ave also had more room in a dedicated Player's Guide to give a lot more guidance to players on how Dark Sun characters would play differently from their counterparts in other settings. I think the FR and Eberron books should have been the model used for Dark Sun.
Overall I'm glad my peeves with Dark Sun turned out to be with the presentation rather than the gameplay or fluff. I was not really happy with how 4E Forgotten Realms turned out ("everyone is dead LOL, except Elminster") so I was skeptical about Dark Sun. I enjoyed the setting in the 2E days and wanted a 4E version to be good. I want new players to get the same enjoyment out of the setting as I did when I first played it. Dark Sun being good gives me a lot of hope for the Ravenloft Campaign Setting coming next year. Dark Sun coincided with the PHB3 and psionic characters (which are integral to the setting) while Ravenloft will coincide with the Heroes of Shadow book and the Shadowfell boxed set.