Not sure if it's still the case, but when I was there us-east-1 was a SPOF for some services world wide. I think if dynamodb went down in the region it was a big, big issue.
The only SPOF of failure I know of for us-east-1 today is the control plane for Route53 - it's distributed and DNS queries will continue to work when us-east-1 is down (including health check based failover), but you can't make any DNS changes when us-east-1 is down.