Microsoft is paying attention, it seems. I'm still going to wait for if/when this actually starts getting implemented. Rocket League -- a game that was already cross-play -- is a good starter but I won't be impressed until I start seeing new games with cross-play between Xbox/PC.
I imagine this is a way to help maintain LIVE subscriptions as the Xbox hardware loses its importance to the Xbox brand.
The real kicker would be to see which features are or aren't compatible in cross-play. Can I voice-chat between Xbox and PS4? Can I send PMs to an Xbox gamer from my Steam client?
Cross-play is a start, but the devil is in the details.