Alright. I'm a hardcore DSer, who has run both the original and revised settings - though I didn't own the original set until years after playing the revised set, so I sort of played it "backwards".
I've also read most of the supplements. Anyways, here's my take:
The Original/Revised split is a big one. The Production values of the second set are better, and they look at things in a more interesting way. They integrate the psionics rules a bit more (the revised 2e psionic system is much better, if you ask me). The revised set is kind of weird in that it never provided a Templar class (a big disappointment for me), and the clerics were still pretty much useless. But rules discussions are probably not what you're interested in.
The differences between the two are primarily thematic - in many ways, the original is "darker", but I don't know if that is accurate. Sure, the revised setting has far fewer Sorcerer Kings, and the Dragon is dead, but that doesn't mean it's any brighter. In the revised setting, there's massive earthquakes and Tyr-storms to contend with, and none of the city-states have become any more stable. In fact, I would suggest that living for the average person has become HARDER after many of the sorcerer-kings died... especially if you happen to live in Tyr, Raam, or Balic. Only Nibenay and Draj escaped relatively unscathed... in some ways.
Some of the states in the revised setting are poorly designed, though: Urik is closed off, which makes it hard to integrate into a campaign, and the new state of Eldaarich is really just a place that would make one or two interesting adventures, whereas the other cities could form the basis of a campaign.
Really, though, the original set gave us a sort of detailed area, and implied that the GM should expand upon it. The revised set gave us eight times the land space that the original had, and detailed it with fantastic locations. My problem with it is that these fantastic locations break the world's feel to me - a plain made entirely of obsidian might be cool, I suppose... but I didn't like it. And I didn't like the Kreen Empire (an Empire of Bugs!? And grass so sharp that it slices your skin, making travel on the ground nearly impossible?). And the windriders of the jagged cliffs/mind lords of the last sea were just too much.
Plus, the revised set goes into the history of athas, in a way that the original does not. THe original set implies that the sorcerer kings were merely wizards who seized power in a dying land, and set themselves up as monarchs. The revised set gives us a story about the making of humanity, halflings originally as oceanic creators, and a magical experiment gone wrong. I don't like the idea that all the sorcerer kings are ten thousand + years old, and the champions of a sort of ethnic cleansing on a huge scale.
That being said, the revised setting does have a few gems. Kurn is a great example - it's a great place for PCs to support, due to it's good alignment, but it's not so great that it dilutes the flavour of the setting. The bandit states are also pretty cool - I like using 'em when I get the chance.
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Now, as for sourcebooks, they're all fairly decent, but I'll just mention a few of the must-haves, and some of the "you should probably overlook"s.
MUST HAVES
The Veiled Society- a great book because it gives you a campaign framework that revolves around magically-active characters. This book has a lot of ideas for running games where the PCs are preservers, rogues, rebels, and warriors fighting against Templar agents. A mixture of politics and skullduggery that fits into Athas PERFECTLY. But, even if you're not into city-based games and want your games to revolve around wilderness killing sprees, you have to recognize that the city-states are vital to running an athasian game... and this book has a huge amount of information on all seven of the main states.
The Monster Books - both DS monster books are very useful. When I ran DS, I was never able to get my hands on the books (I still only have them in PDF form), and it really hurt my game. I found myself scavenging for whatever monsters I could get my hand on, and every time I ran an adventure, I kept wishing for stats on Crodlus and Mastyrials.
Many of the monsters are kind of... I dunno.... "over the top". But it's worth taking a look at this book, because there's enough to at least get the ball rolling for your own inventions. And a fun aside: in original Dark sun, there's a lot of mention that mutations prevail in the world. Single, one-of-a-kind monsters abound in the campaign setting, meaning there's never a reason you can't use that recent gem from a product you just bought.
The Ivory Triangle - Technically, I don't own this, but I have read the whole thing back to back, and I think this is the best place to base a campaign in Dark Sun. We have two city-states at war (Nibenay and Gulg) and a very detailed slave tribe (Salt View) that tries to profit off this situation, while maintaining their freedom. REally,, throw PCs into the mix and you have instant fun.
I borrowed it from a friend, read it cover to cover twice, and wished I had a chance to run it before he took the books back. I still search for a print copy from time to time.
MISSES:
Dragon Kings: Actually, this is a great book if you're playing a 2e game. But if you're trying to run 3e games, there's little that you can use in this book. It's really just a splat book with a DS label on it. Much of the material on Avangions and Dragons were later detailed in Defilers and Preservers (not a great book, either) - whether they were reprinted or actually rewritten, I don't know.
Valley of Dust and Fire - I don't like this book because it's a self-contained campaign area that is rather difficult for PCs to reach, and because of little things like iron being common in the region and the entire city-state being ruled by THE dragon. Really, though, this could be a fun book for some gamers. It's written for higher-level play, which is probably why I never used it.
Mind Lords/Windriders of the Jagged Cliffs. Both books are bad, but for for different reasons. Windriders is just a bit too pulpy (flying on hot air balloons, with living organisms that have semi sci-fi uses!), and it has a swamp as a primary adventuring locale (and swamps just don't scream Dark Sun to me). It has it's cool parts, and I would absolutely LOVE a windriders campaign if it was set in it's own universe, but as part of Dark Sun, I don't like the book.
Also, it suffers from something they did far too much in Dark Sun - they make an area that is very hard for PCs to get into or leave (much like the Slave Tribes book - once you join a slave tribe, you can't leave, which can make the book difficult for DMs to use in actual play). As a result, it's actually easier to play a campaign where everyone's a halfling than it is to play a Dark Sun campaign with half-giants, Thri-kreen, and what not.
Mind Lords is even less Athasian. There's a sea, and people have boats on it. There are athasian dolphins, that are actually quite intelligent. And the mind lords, pretty much a trio of psionicists that proclaim themselves gods in Big-Brother fashion, are kind of generic terror lords. And this is another setting that is hard for PCs to enter.
The place is set in a large grassland that would be bleak in most campaign worlds, but in Dark Sun is pretty much a heaven. And then there's a huge sea that provides an abundance of food. And many of the locales seem much more medieval than Dark Sun towns. Really, it seems more like a book based around psionics that someone later retconned into DS than any actual Athasian product.
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Regarding Adventures: Don't buy 'em. I've only run a few (The Road to Urik, Arcane Shadows, and another one whose name I forget), and played in Dragon's Crown... and they all were lacklustre. I remember one of them being based around a Preserver trying to become an Avangion, and the PCs helping out. And it just had nothing in the way of exciting scenes. Really, every time I ran a published DS adventure, it fell flat on it's face. Plus, they had a bad habit of giving out way too much treasure (one of the boxed sets had an entry-level adventure that gave the PCs a metal longsword +1 that had some charged effect on it... considering how the PCs started out with weapons that were fundamentally -1 weapons, that would be like giving 1st level PCs in the Realms +2 Longswords on their first adventure!)
The adventures in the Original and Revised set are also fairly bland - the original has the PCs starting off as slaves (don't do it, it's been overdone) and trying to find their way to safety, and then getting caught up in something for no real good reason. The Revised has a bit more context behind it, and starts off alright, but then gets really weird, with the PCs visiting a ruin from the Green Age and coming across Halflings with grafted-on biological devices that are just plain silly.
