Can someone summarize what makes Al Qadim standout? It's something I totally bypassed during my 2E days, but I see it come up here occasionally as a fan-favorite. I'm interested in both mechanics and tone.
Mechanically, to me at least, the one thing that really stood out was that it fully embraced the 2e kit system. All PCs were expected to take a kit, and there were a lot of very flavorful kits in the setting. The other thing was the sha'ir class that relied on having a little 1-foot tall genie you would send off to retrieve spells for you. Fun idea, but I was never super-keen on the implementation of it (you need a lot of planning ahead, because you could, for example, get even priest spells, but sometimes you needed to wait hours for it).
However, I think what makes it such a fan favorite isn't the mechanics, but the flavor of the setting. It was pure Arabian Nights-style fun. Genies, sultans, pick pocket thieves going on epic quests, or my personal favorite - the religious assassin.

It was abounding in fun adventure possibilities, and with the strong use of kits, it hit a nice balance of "flavor impacting mechanics" without having to create drastic new mechanics like new classes and such.
But that's just my recollection from 15 years or so ago. I'm sure others will have other opinions of what made Al Qadim great.
As for the original topic, my money is on Dark Sun, but I think "New setting" would have been a good option to have in the poll. I can see it really going in any number of ways, but if I have to pick, I'd go with Dark Sun. And I'm fine with there being some changes, like including all of the core races in some form - and given how strange Dark Sun is, I don't think it would be hard to retrofit them in unless you are a canon purist.
But I guess part of that is that I prefer "This is a new edition, and we're saying dragonborn have always been in the world, just maybe over those mountains or something. Whatever." rather than another Time of Troubles/Spellplague world-altering explanation of the changes. Considering it hasn't been actively published in a very long time, and the number of groups that will convert an existing campaign to 4e Dark Sun rather than start a new one is probably exceedingly small, I would prefer a "It's just different" than having some grand explanation of the changes.
I would really hate to see some convoluted the gods return to Athas, try to re-shape the world to better match other worlds they oversee, only to be driven off by an unholy alliance of an escaped female mul gladiator and Borys reborn who give birth to the dragonborn race, while elves take to the newly grown forests and reclaim their heritage as eladrin and then teaching the halflings literacy and proper hygene (and focusing on their psionic surfboards because that was the awesomest part of Dark Sun in one of the designer's high school games), but it's all 12,000 years later, so none of that actually matters, and on and on. Sometimes trying to explain the difference just makes more of a mess than saying, "Hey, it's different now."