When Mortal Kombat came out, it had a unique visual and style, with its digitized sprites and BLOOD. Now, if you actually go back and watch some early MK1 videos, the amount of blood compared to almost any M-rated game nowadays is actually quite laughable, but this is 1992 we're talking about it! The gameplay itself doesn't have a lot of depth; almost all the characters had the same normal moves, speed, height, jump distances, etc. Which also means it was very accessible to the casual arcade goer to just pick their favorite character and go at it. So accessibly + fun, never seen before aesthetic + controversy/aka free marketing = popular title.
The next two sequels did improve upon the gameplay, making it faster and smoother, giving characters individuals chain combos, run buttons, interactive stages, and improved animation. The game's unique "Big Trouble in Little China" crossed with popular Western mythology still gave it a unique atmosphere and character design, and the game's humor picked up as well(although some feel went a bit too far with all the Friendships and Babalities). Of all the pre-MK9 games, UMK3 was commonly held as the best, and was played at tournaments for quite awhile.
After that...it gets pretty shady. BUT, MK9 came out and apparently the gameplay has improved quite a bit(although I haven't played it myself), almost every gamer in the 18-25 range has a nostalgic enjoyment for Mortal Kombat(nostalgia has a funny way of remembering the good parts and not the not-so-great gameplay parts), and it still carries the same appeal it always does: cartoony, accessible violence with fun characters and unique atmosphere and art direction.
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It will launch for $350 with some Wii Sports s***. I bet my life on it. Damn, that KoolAid guy is awesome - NGamer64