Actually you probably should add in a team of admins unless you assume a single person is capable of 24/7 service and handle the possibility of multiple large-scale issues simultaneously. Also factor in expertise (not just familiarity) in all areas of infrastructure.
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.
