Clemens has makes a good point here ... I can't help but think: COM Type Libraries, anyone?
It looks very sweet, and I'm dying to get my hands on some Indigo bits to play with. My biggest fear, whoever, is how people will understand the programming model. I totally agree with Tim that writing code first is a very productive way of writing your services implementation (and one I do all the time with ASP.NET Web Services) as long as you fully understand the contract and schemas you're expressing in code (which, with isn't too hard once you've grasped the basics of the XmlSerializer).
However, I know from experience that code-first models such as ASP.NET Web Services (and possibly Indigo, from what little I know of it), blur the line between what is just rpc-like code and the real messaging/loose coupling/XML-based underpinings of it all. Way too many times I've had to go through real trouble to explain someone just what it means to write a [WebMethod] with a ref class as an argument :(.
So, in that sense, I'm thinking about both how nice the programming model is going to be, and whether it will confuse more people on the issue ;)
 And Tim has an even better reply here.