You need the admins either way, AWS does not help with that. Most problems from experience are with the application (Database, Memory, Locking, ...) not with the infrastructure (Load balancer, Linux, ...).
[Edit]: The problems Reddit, Digg, Foursquare had were problems with application code and application infrastructure (MongoDB, Cassandra, RabbitMQ, Redis, ...). AWS admins would not - and have not - helped with that. So you need your own admins, AWS does not spare you those.
[Edit]: The problems Reddit, Digg, Foursquare had were problems with application code and application infrastructure (MongoDB, Cassandra, RabbitMQ, Redis, ...). AWS admins would not - and have not - helped with that. So you need your own admins, AWS does not spare you those.