The understanding is that the services which you use in "global" region are hosted in us-east-1 .. CloudFront, Route 53, IAM. So outages in us-east-1 can cripple a lot more than you'd think. Your EC2 proably still runs but I would not count on AutoScaling or being able to log in and do admin tasks. I'm sure AWS has also thought about it and the good part about us-east-1 is that it has 5 or more zones so it's harder to take it down.
Certain API endpoints are not considered part of the availability guarantees. You might be able to verify and create IAM tokens, but not create new IAM roles, new S3 buckets etc. AWS sometimes advises that these operations shouldn't be done automatically, but not everybody listens.
