I work with ASP.Net MVC primarily (since v1), but I think Play framework have a lot to offer in the JVM web development space.
The biggest key selling point for me is the 'hot' reloading: make changes in your code, refresh your browser, and see the changes immediately. You don't need the whole project to be rebuilt. As far as I am aware, this is what makes Play unique. The 'hot' reloading is probably the reason why it's useful to show nicely formatted errors in the browser (during dev / debug mode), you can see immediately where you stuffed up soon after browser refresh. Errors are also pushed to the console.
However, I realise that ASP.Net MVC View templates can be updated during debugging session, I only wish to be able to do the same with the .Net classes without having to rebuild it.
Plus, being on the JVM you'll have plenty of PaaS deployment options for cloud hosting provider (e.g. Google App Engine, Heroku, OpenShift).
With ASP.NET if you are in a 32 bit machine (who still is?) you have edit and continue so you can do the same as in Play. AFAIK they are working to bring that to 64 bit machines too.
Don't understand me wrong. I see a lot of value in Play. My only beef is with the "this the awesomest thing you'll ever see" approach to selling it.
"One of the favorite features in Play" is being able to show errors in the browser and not in a log? What year is this? 1995?