I think the Plaguelands are probably the very reason the Scourge went after Darnassus/Orgrimmar. In the Eastern Kingdom the Forsaken, Blood Elves, Argent Dawn, and Scarlet Crusade are all mobilized against the Scourge. They are constantly on guard against the threat from the cold north. In northern Kalimdor, there's no great threat other than some demonic remnants from the Legion's invasion. The Horde and Alliance forces would have their guards down.
I would think that Teldrassil has some defenses to it; the druids grew a giant tree, surely they could cause the tree to, say, fire giant thorns or something. Not to mention that the Night Elves already have an excuse to hit up Northrend: early NE civilizations were there, and they grew the World Tree Vordrassil there.
The (apparent) excuse for razing Orgrimmar is so Blizzard can rebuild it in a better, newer fashion. But Orgrimmar is a major hub for the Horde, so that's fine. Darnassus is not a major hub for the Alliance, there's no particular reason to rebuild it other than to waste WoW designer time. If anything, I'd rather that the developers rebuild Gnomeregan (the instance, not necessarily a new city, although that would be nice too).