At this point, there’s very little difference between the Xbox One and the PlayStation 4. But there was one key differentiating point. Xbox gamers could only play with Xbox and PC players as Microsoft was restricting access to the multiplayer component. Microsoft just announced that game developers can now create cross-platform multiplayer modes that work with other consoles and operating systems.
So it means that the next Call of Duty or FIFA could feature a multiplayer mode that works with both Xbox and Playstation gamers. It just depends on developers now.
Microsoft has historically restricted cross-platform play as the Xbox Live was the first successful multiplayer network for consoles. The PlayStation Network only appeared with the PlayStation 3.
All in the lap of the game developers now, not Sony
From what little info is out there so far, the Xbox Live crew are giving over the compatibility development to the game developers. Sony apparently does not need to be involved for PS4s to take part.
I think it remains to be seen if Sony will likewise open up their own gaming network to Xbox devices, or push towards some new formula for the whole industry like a joined network or automatic compatibility.
Most online games do player match making based on player ratings. It's often like a chess rating system where your rating goes up or down based on the ratings of the players you win or lose to.
So, if mice make FPS players "better" (and they do), then really good console FPS players would get matched against poor to mediocre keyboard/mouse players, and at the high end of the ladder it would almost exclusively be keyboard/mouse players.
I think the current player rating systems would be sufficient to prevent games from being a complete blowout. At least once it's figured out what a given players rating/rank should be.
Sony’s response, issued one day later to GameSpot, suggests the market-leading platform holder remains open-minded about cross-console play but stopped short of discussing a deal with Xbox Live."PlayStation has been supporting cross-platform play between PC on several software titles starting with Final Fantasy 11 on PS2 and PC back in 2002,” reads the statement from the corporation. “We would be happy to have the conversation with any publishers or developers who are interested in cross platform play."