According to Colin Percival on Twitter[1][2], the US East-1 AZ has more IP addresses, and thus probably other resources, than the rest of AWS put together. It casts comments about "limited to one availability zone" into some relief.

[1] https://twitter.com/cperciva/status/219067641023840257 [2] https://twitter.com/cperciva/status/219067963356098561