The Revised boxed set was most definately the high point of Dark Sun.
Perhaps the single best thing about it was that it included a booklet of the new psionics rules, since (from what I heard, not having it) the original boxed set required you to have the Psionics Handbook.
That said, the Revised boxed set just offered so much more potential. News on the city-states where their sorcerer-kings were dead, and what could possibly happen there, the terrors of the Obsidian Lands to the south. The mysterious and new city-states to the north, and the Kreen Empire to the west. Not to mention that the race selection was expanded to include aarakockra and pterrans. It just offered so much more than the original boxed set ever did.
The only things I disliked about it were the loss of the Templar class and that defilers suddenly defiled when they memorized spells, as opposed to when casting them.