Downsides of course - the development tools are MS only, and even with Microsoft's pervasiveness it is going to take a while for the critical mass to install the browser plugin - currently available for a handful of browsers for PC and Mac, not yet (ever?) for linux.
I was hoping to see a few more components provided in the box for common functionality such as form fields. I was under the impression that Microsoft would try to take advantage of silverlight's .net underpinnings and sell Silverlight to hordes of Visual Studio developers by creating a library of form elements like you would find in a typical visual studio project. Third party components do exist, but I was surprised not to see it built into Blend.