CharlesGrey: The fact that those Steam box consoles ( PCs? ... whatever ) are supposedly going to run some version of Linux is going to be a factor. And then there's store loyalty -- most gamers seem to prefer for their collection to be in one place, which is part of why Steam is so popular and keeps growing, and it's hard for any other store to compete.
It will be a factor if they take off, which is a big gamble right now. They aren't really predicted to, but who knows. Honestly Valve dropped the ball and is taking to long to get them to market in my opinion. They missed their window with Windows 8. It is true that gamers prefer their games to be in one place (mainly Steam), but I think we can all agree that that is not realistic anymore and will slowly change. EA has Origin, Blizzard has their client, MS has the Store now. Pretty soon it will be harder to ignore other clients.
CharlesGrey: What that MS store would really need are exclusive games, and not just a handful of them either. No one already using ( and enjoying ) Steam, GOG or other services is likely to make the switch for games they could just as well buy at their current favorite distribution platform. And since major exclusives are a huge investment, I'm not if MS is interested in taking that route.
In fact, you know how insanely commercially successful all those crappy micro-transaction infested mobile games are? I could see MS being perfectly content with focusing primarily on mobile and casual style games, and even making a bigger profit that way, rather than trying to compete with Steam for the "core" gamer audience. In a way, competing for GOG's audience would be even harder and less profitable for them, since we're such a special, old-school niche bunch around here. So no, I still don't really see them as direct competition to Steam ( at least at this time ) and even less so for GOG.
Well Fable Legends, Gears of War, Sea of Thieves, and Halo Wars 2 seems like a good start as far as exclusives on PC go and no doubt there will be more. I honestly think they will try to focus on both, that is the entire idea behind universal apps really... they will work on everything. No mater if the mobile games or hardcore PC games. Granted we will see more mobile games on PC than hardcore PC games on mobile simply because of hardware and controls.