WARNING: A lot of Play/Scala cheerleading from a former Ruby developer. I can't help it. I love my job. Feel free to summarily disregard. ;-)
Play is an evolution for sure. But I'd consider it "beyond". Validation concerns are where they should be: At input. Not the Models. Comet, Websockets, etc are all trivial. Chunked Responses FTW. You don't need different responders. Render _anything_ by having a Writable in-scope.
Our API calls to render our models as JSON look like this:
api.photos.get(id) map(Ok(_))
The plugin universe isn't there yet, but I tend to think most apps out-grow most framework plugins pretty quickly.
Honestly, I find Play/Scala _more_ productive than Ruby MVC in general. Fewer bugs. Much better performance and memory usage, so fewer constraints (that one is huge). Rework is much easier, but also much less necessary. The language itself is a joy (IMO) to work with.
The Java ecosystem is a huge enabler. You can have complete control over resizing images in ImgScalr, faster, with no external dependencies, in less code and a flatter learning curve than anything I've ever used in Ruby. Scala itself presents a lot of new things to learn and get comfortable with language-wise until you feel you're back in the groove, but once you've got it, I've found that the ecosystem makes the complex things much easier, and the language itself makes the easy things engaging as well.
Ruby doesn't really help much trying to tackle hard problems IME. Image Resizing? Better shell out to ImageMagick. Dubbing a video? Hello shell my old friend. Cache coherency across a cluster? Fire up another external service (maybe Redis) to manage and start working through failure modes and failover scripts if you care about availability. Compare that to Hazelcast. Need to serve static assets? Better setup an NGinx proxy. In-memory near-caching? You don't have the memory for it, and the GC can't handle it anyways, stick to Redis.
Put a crew of experienced Play developers head-to-head with experienced Rails developers, give them the same goals (and non-goals), and you'll have a faster, more maintainable solution from the Play team in the same or less time for anything but the most trivial apps.
IMO. :-)