Lots of discussion pertaining to MK9's gameplay, but those topics aside, MK9 in it's entirety was a direct fan service. That game was literally every MK fan's fap dream. Mine included. Every Trilogy era character was included, which are the majority's fan favorites. Numerous nostalgic throwbacks in fatalities, stage fatalities, arenas, Easter eggs, and music. Plus an interesting (albeit flawed) new story telling design.
Couple that with grand redesigns for every character, the glorious return to the 2D plane, and the ability to finally create your own unique combos using the new mechanics instead of 18 button long dial-a-combo bnbs and you've got a hella fun game with new perspectives on interesting characters as well as a fresh new gameplay design.
Also worth mentioning MK fans were ecstatic to wash the foul taste of Armageddon (and MK vs DC) out of their mouths.
MKX and MK9 are more similar than some people realize. Each have their own pros and cons as with any game, but a lot of the stuff MK9 was appreciated for is overlooked in MKX simply because we've seen it before and it no longer has that "new car smell". Diversity in character gameplay archetypes, core mechanics, unique story mode, and a new online experience (I didn't say good
) to name a few.
I loved MK9, but imho MKX is the superior title and only falls short when it comes to arena selection, character roster, *music, and a few gameplay nitpicks.