Therein lies the rub - if you are working with AWS' key-value store, and then also their Lambda system, then you don't really develop server side - you develop everything client side (mobile app) and just use the cloud as a dumb logic store.
Moreover I feel Lambda can do pretty good stuff. I am sure it will integrate with other AWS services pretty soon and once there is connectivity to an RDS instance, then it solves the use case for half the developer population out there.