Adding up all the traffic jams in World of Warcraft would probably make an insightful list-- I can remember some crazy days in Ironforge, a huge mess near the battlegrounds when they first opened, a lot of clustering during the AQ World Event, and anyone who logged on in the early days of Burning Crusade knows that Hellfire Peninsula was a laggy mess as well.
But Blizzard claims, once again, that they've finally learned their lesson, and in Wrath of the Lich King, things will be different. Shattrath will be designed to stay up in the air (even if landed on by thousands of players), and when we played the expansion at BlizzCon earlier this year, we already saw one way Blizzard plans to split the load: Horde and Alliance players are starting at different zones on different sides of the continent.
I doubt things will be perfect-- it seems like players always think of some way to ruin even the best laid plans of Blues and men. But we can say this about Blizzard-- they are visibly learning from every iteration they release. It'll be really interesting to see what they've come up with to improve upon the expansion experience in Northrend.