I was thinking this morning that one way to create a service of real value is to ensure that the way a user experiences that service is commensurate with the value you are trying to deliver to the user. Feels like a simple idea but harder in practice.
For example, Foursquare. Stated mission: find your friends. With the mobile app, iphone version at least, for a user to get that benefit - finding their friends - requires only two taps of the thumb. Nothing more. Which makes sense - simple user experience here not only works perfectly but is in fact consistent with the initial effort required for a user to have a successful interaction with the service. Of course, there is much more richness to the application - but the initial, baseline experience is in line with the benefit of the services' goal.
Which doesn't mean that simplicity in itself is the answer to the design of users experience. Contrast Hunch. Stated goal: customized recommendations that get smarter the more you use it. To get those recommendations requires some effort by the user - there are a number of user touch points - questions, search, likes/dislikes. As I see it, Hunch does not shy away from presenting to the user a bunch of inputs and experiences. Indeed, the services' value proposition (better recommendations) is of such high, personal value that it implicitly and explicitly requires a deep experience for the user to get that value. Again, an experience consistent with the stated mission.
Finally - Dailybooth. Really only two things to do over there - take a picture, or comment on a picture. But that's precisely the value proposition: document and share your life with others. The experience and functionality are consistent with the value a user gets.
So while I am able to check in to Foursquare while at breakfast and see who else is there, all in 5 seconds with only my thumb, I am going to spend 10 minutes back at my desk figuring out what pair of headphones to buy. And later on spend a few minutes catching up with Jon's life, in pictures and words.
To me, the key innovations of these three services, to use just a few examples, are simply in how they present user experiences (and the design that comes along with that) that are completely consistent with the value their users receive from experiencing the services that way.