Personally, I'm partial to developing a simple but powerful services oriented back end API, particularly when you might want multiple clients. I think the advent of excellent JavaScript MVVM and micro- MVC frameworks makes this a natural choice, as writing application code using BackboneJS, KnockoutJS and others allows for a capable development environment.
Given that you're planing on putting application logic in the client, WebAPI would be a good choice for this, as would the open source project Service Stack. I recommend Service Stack, as I've been using it for the last few months and have found it to be an excellent replacement to WCF (which I used extensively in the past).
The group maintaining service stack has posted an example application using the framework to develop a StackOverflow like clone which shows a development pattern that I believe is especially compelling.
I'm currently writing a tutorial series on service stack on my blog.
Coincidentally, Stack Exchange hired the original Service Stack author. He continues to commit actively to the Service Stack project.