When Street Fighter 2 was all the craze, I saw Mortal Kombat on an arcade, nobody was playing, I checked the (bad) digitized characters with that jerky, laughable animation (really few frames of animation), and everything looked so stiff... so I just moved on, considering it a B-grade fighter, trying to cash-in on the fighting game genre (wildly popular those times)
some weeks later, on an Arcade which had bigger displays (and thus, it was more popular) I saw a crowd, and I was curious, so I went in to see the game. It was the same shitty game as before, so I wondered why?
Until I saw sub zero head and spine ripping fatality.... My reaction was: "OMG, did I just saw all that gore on an actual arcade game???

" And that was it, THAT got my attention.
If you think about it, fatality is THE main thing of this series. Without them, it wouldn't have been as successful, since it had mediocre gameplay, terrible animation and lame backgrounds. But the violence made what the franchise is today (duh! I know)