One of my favorite features in the new WCF Web API’s support for Media Type Processors. WCF Media Type Processors offer a really powerful way to build some sophisticated services with minimal configuration – or code, for that matter – because they leverage the media support and content-type negotiation features that are built into HTTP.
Over the years, I've had a tough time with Windows Communication Foundation, otherwise know as (and sometimes cursed as) WCF. I knew it was what I was "supposed" to be using to because it handled complex scenarios like managing access as secured messages passed through systems and users with different access rights. However, it didn't seem to be able to handle my simple scenarios – things like returning very simple, unrestricted information from a server to a client – without requiring hours of pain, configuration, and things like writing a custom ServiceHostFactory.