I am referring to DNN's Services Framework WebAPI. DNN doesn't use WCF and I didn't mention WCF. Not sure why you mention WCF. 

Actually DNN 6.x used an MVC based service framework (http://www.dnnsoftware.com/wiki/page/...), whereas DNN 7.0 uses WebAPI. There are some differences so generally speaking it's easier to start with WebAPI as the MVC one only existed for 6.2.0->6.2.9. As to things you can do, there isn't any real difference in the DNN implementation - however WebAPI is much more extensible and has many more integration points (e.g. I can add a request filter, formatters, logging etc. all easily in WebAPI but it's a bit tougher in MVC - primarily as WebAPI is unsurprisingly designed for service requests whereas MVC is more generic). Natually you can use a component such as WCF which sits at a different level and will work in all versions -the counter argument to that is WCF is a bit unwieldy/slow as it's much more powerful (in many ways WebAPI is simply the http web focussed portion of WCF i.e. doesn't come with lots of soap, binary etc. support that you probably will never need built in.

By the way I'm guessing you are asking this as you're writing a module and want to know what version to target at. We have quick links to developer notes on the wiki at http://www.dnnsoftware.com/wiki to allow you to more objectively decide what edition you target e.g. if you wanted to use DAL2 (petapoco) you would target 7.0 or above.