The stuff that's exclusively hosted in us-east-1 is, to my knowledge, mostly things that maintain global uniqueness. CloudFront distributions, Route53, S3 bucket names, IAM roles and similar- i.e. singular control planes. Other than that, regions are about as isolated as it gets, except for specific features on top.

Availability zones are supposed to be another fault boundary, and things are generally pretty solid, but every so often problems spill over when they shouldn't.

The general impression I get is that us-east-1's issues tend to stem from it being singularly huge.

(Source: Work at AWS.)