X-rays are supers. It's just another attack that costs meter. They cost 3 meters, has character specific properties, and can't be broken with a breaker. Thats it. What your character's x-ray does is an important tactical decision.
I.e. Kung Lao's x-ray is a really good anti air that has a little bit of a bigger hitbox then his normal spin. It also has armor, easy to hit in combos, and after it hits you can dash and combo 2,4,1+3,(JF)2.
Also, doing an x-ray at the end of a combo isn't the most efficient way of doing damage in mk. For a lot of characters, it's better to use EX moves then an x-ray in a combo. It saves meter, does about the same damage, and lets you keep meter to use a breaker to regain momentum if you start getting hit.
Fatals are just added in for both nostalgia and is basically ingame teabagging. It's completely optional
But Yahtzee has yet to play a fighting game except SSBB. The reason I say this is because you need other people to actually play a fighting game. They are multiplayer games. Period. Judging a fighting game on its single player and story alone is like judging chess for the same reasons. They aren't designed for that purpose and it is silly to judge them that way.