Dark Sun would have been a disaster from a business point-of-view. A new ruleset, particularly one that so profoundly differs to its predecessor, needs a fairly standard setting.
I agree with other posters who suggested that PoL-land should have been the first campaign setting released: a fairly generic fantasy setting built around the 4E ruleset.
(I actually really like the post-Spellplague Realms and the versions that came before it. However, WotC should have actually spent some money on a map rather than going the bum-wipe route.)