Actually, server crashes would be a thing for content such as that, but I feel that is something SE needs to work on. At the same time, though, I would absolutely LOVE to see something such as the Uldah raid mentioned above.
Imagine: Everyone relaxing in uldah, suddenly, a horn blows and Uldah NPCs start running to their rooms while the Uldah guards ready their weapons. Empire forces begin flooding into the city in waves. The first wave is small, probably levels 30-40. The second wave is slightly larger, with levels around 35-45 with siege weapons. The third wave could consist of levels 50-55. The 4th wave could consist of levels 55-60 enemies, with siege weapons and a "boss" at the end of the wave who has the ability to summon "reinforcements" to aid him as he attempts to control the city.
This would, of course, require the ability to make alliances with other parties, but something like this is what I want to see.