Mortal Kombat X is out, and if you direct your attention to its Steam page you'll notice that the user reviews are not particularly positive. There are plenty of recommendations, but much of the text is scathing sarcasm aimed at the fact that the game won't actually run. Polygon has posted an explanation for the problem, which arises from Steam's "streaming install system" that's intended to make the game playable while the installation process is still underway.
Players firing up Mortal Kombat X for the first time are greeted with a message telling them that the game uses the streaming install system that allow them to play while additional content continues to download. A message to the same effect is up on Steam. "As soon as the game is played for the first time, a batch of DLC content will appear in your download queue," it says. "Check the Steam client to see the download progress on DLC items labeled Mortal Kombat X Install Pack 01 through 23. You’ll be able to play the game while these downloads complete."
The problem is that these install packs don't appear in the download queue, and apparently they're not downloading automatically at all. Without them installed, players can select a faction and horse around with options, but trying to start a fight results in a crash to desktop. The solution is to go to the MKX entry in Steam, where the packs are listed as DLC; click the checkbox next to each of them, and they'll start downloading.
You'll be able to play once the required downloads are done. One of Polygon's screens indicates that the extra files clock in at just under 18GB in total.
Update: A message acknowledging problems with the installer is now up on Steam. "We are aware of download issues with the game and working closely with Steam to resolve the problem," it says. "We will provide an update as soon as possible."