I have been waiting for this announcement for some time - that Microsoft would allow people playing on multiple devices to play together - because it utilizes the main strength of Microsoft's brand which is market share. The PS4 may have outsold the Xbox One so far, but when you add in PCs they touch almost every house in America. And if you game on a Mac you probably have a Windows mode because Apple isn't compatible with most things they didn't design. Microsoft can make this statement knowing that they can pair with their PC products without help from anyone else, and if PS4 or Nintendo's next system doesn't play along they are the bad guys. It also puts the onus on game developers to focus on making the Xbox version so they can get that cross promotion synergy boost with PC. I am curious to see how Sony responds, and which games are the first to be playable between the two rivals. As for PC the first title announced was Rocket League which ought to be fun.
Besides the great combining of friend groups, this does bring about a new world of cheating/fairness accusations. PC people have more capability to modify the game files and maybe tilt the matches in their favor, and it is possible that games may run better on one system or the other. I imagine the growing pains will be frustrating at first, but if this cooperation takes off it would be exciting to see where this takes us over the next decade. It be much easier to fill up servers and game modes across multiple platforms. Personally, I would love just the ability to chat across platforms while gaming as not all of my friends have agreed on a master system or have the money to get them all, or get them all together to play some classic 6-player X-Men.