The Burning Crusade's launch was encouragingly issue-free when it first went live, and early indications are that Wrath of the Lich King will follow suit. There has been no post-launch downtime as of yet, and the servers appear to be holding up fine.
On the other hand, patterns that did sometimes manage to put the brakes on the grind seem to be manifesting yet again. Namely, the frustrating consequences of overpopulation. Just like in The Burning Crusade, certain quest monsters are scarce and in high demand. The general rule is, "if your mod isn't on a 30 second respawn timer, you might want to do something else until off-peak hours." The fact that Wrath of the Lich King offers two starting areas for level 70 players in Northrend does mitigate this a little, but you'll encounter the situation sooner or later.