I really like the Dark Sun books, so much that I am constantly rereading them over and over. I just started a new DS campaign also.
I just assume that Dark Sun is so good because they already had practice with the other worlds. I like what they did with Eberron, and enjoy the setting for my other campaign. I dont really see any difference between 3.5 Eberron and 4e Eberron, which is good in my opinion. I am not a fan of FR 4e, but its not something I get worked up over. I just dont personally like it, but would still play in it. So when it came to Dark Sun, I figured they learned a lot by creating the other two settings for 4e, and thats why Dark Sun came out so well. I admit this may not be the case, and Dark Sun may have been just as good being the first setting, but I think it helped personally.