As Tim Sweeney pointed out not long after Microsoft's announcement, UWP does bring with it a number of big caveats, including one that involves Microsoft having far too much control over that particular ecosystem. If services like Steam and Origin are not able to sell UWP games, the biggest game Microsoft could be playing is the market.
As it stands today, Microsoft's stance hasn't changed on UWP, and it will continue with the rollout of select games. While we'll see a customized version of Forza 6 in the future, and the cross-platform Quantum Break next week, Killer Instinct is here right now, and it happens to be "free".
Quotes around the word "free" when it comes to gaming can only mean one thing: in-app purchases. While that's true with this particular version, it seems that the base game itself still has a ton of gameplay to take advantage of, and if you get so into the game that you want to haul out your credit card, you will be able to enjoy content parity with the Xbox One version.
One of the biggest perks of Xbox One games coming to Windows 10 is cross-platform play - Xbox One gamers can play against PC gamers. Microsoft isn't oblivious to some of the wants of PC gamers, either, as it touts that 4K resolution is fully supported, and because we're talking about PCs here, 60 FPS can be expected (that’s if your hardware is up to snuff, of course).
In the "Season 3" paid DLC release, players will be able to play as Kim Wu and Tusk The Immortal Barbarian, as well as a couple of unique entries: Rash from Battletoads and Arbiter from a little game called Halo. Over the course of the next few months, more characters will be added to the roster. Along with Season 3's release, the game has had a bit of a graphical makeover, no doubt to help improve the visuals on the PC. If you want an example of those upgrades, you'll want to hit up one of the new fighting stages, which includes the "Arena of Judgment", but previous levels have been improved as well.
If you love Killer Instinct and don't want a crippled game, you can purchase the full-blown package for $60 USD, which includes Season 3 content. $40 and $20 versions are available as well. For those really particular about the fighters they play as, $5 a-la-carte options will be offered in the future, as new fighters get released.
As mentioned above, the next Xbox One to Windows 10 game we'll see launch is Quantum Break, due out next Tuesday.