This isn't far from the first such service. Akamai, CloudFlare, and Incapsula all offer DDoS mitigation services. AWS Shield, however, is only for AWS customers.
What Amazon brings to the DDoS battle-line is the sheer scale of Amazon Web Services (AWS). This family of services makes up the world's largest public cloud.
Werner Vogels, Amazon CTO, in announcing Shield at AWS re:Invent, claimed, "I think this will really help you protect yourselves even against the largest and most sophisticated attacks that we've seen out there."
I wish them luck with that. Even AWS might shake some with an assault of the magnitude that took down the Dyn Domain Name System (DNS) provider earlier this year. 1.2 Terabits per second (Tbps), which is estimated to be the attack's high point, would be enough to wreck anyone's day.