That's a good question, and as of Patch 3.3, we're about to become far more intimate with it. At present all we know about Quel'Delar is the area near the Argent Tournament called Quel'Delar's Rest (as seen above), where a massive dragon skull seems impaled on a sword half buried in the ice. Speculation on the sword, the thing it is impaling, and whether or not we'd get to pull it out of the ice (as well as to links to Quel'Serrar/Quel'Zaram, famous swords with the Quel prefix meaning High in Thalassian/Darnassian) look to be on the verge of being solved.
It seems that MMO-Champion has found a whole series of spells related to Quel'Delar and the new Icecrown instances in the latest PTR build. You apparently need to bring the blade into the Forge of Souls and temper it, then bring the tempered blade to someone, possibly Krasus, which then possibly phases you into a battle to cleanse an evil form of the sword. Please keep in mind this is all speculation based on spell descriptions, but at the very least we know that there's more to that patch of barren ice near the Tournament than we'd been led to believe. It's apparent from the descriptive text on this spell that Quel'Delar is deeply connected to Icecrown Citadel.
Is Quel'Delar eventually going to be a player item? Is it connected to Quel'Serrar, as was speculated? Does it want to help or hinder the Lich King, or is it connected to some other denizen of Icecrown? We don't know yet, but the time till we find out seems fast approaching.