I'm not sure if I'm late to the party in realizing that MK has negative edge. I was just messing around in practice, and sure enough, it does.

For those who are not familiar with this, below is how it works:

When you press a button, say X (or A for Xbox), the command is recognized immediately. So you can perform Sub-Zero's Ice Ball by inputting Down, Forward, X. Negative edge is when a command is recognized upon releasing the button after it has been held down for a certain period of time. So, with negative edge, you could perform Sub-Zero's Ice Ball by first holding down X, after a few second, press Down, Forward, and then release X--the Ice Ball should come out.

It can be used at times to simplify complex combos, but depending on the character's move set, you may or may not be able to use it. The reason it may not work involves moves with the same directional input as the move you are charging and when you intend on releasing the charge. To test this, you can use Jax as an example:

Jax has the:

Dash Punch - Down, Forward, Triangle (Y for Xbox)
and
Gotcha Grab - Down, Forward, X (A for Xbox)

Both of these moves have the same directional input. So, let's say we want to use negative edge to activate the Dash Punch. We would do this by holding in Triangle--and later pressing Down, Forward, and then releasing Triangle. Now, do either his Ground Pound Close (Down, Back, X) or JP-Pop-Up, Dash forward and press Triangle, Square, and then Dash Punch. It should connect just fine. Now, try the same combo, but using negative edge to trigger the Dash Punch--so Ground Pound, dash, press and hold Triangle, then press Square while still holding Triangle, followed by Down, Forward, and release Triangle. Instead of doing the negative edge Dash Punch, he performed the Gotcha Grab. This is because the final button pressed before negative edge also has a special move with the same directional input. So you will have to test each character and move to see if it can be used in combos.

Wow, I'm looking up at this wall of text....sorry for the long post, I just started typing and didn't notice how long this became--anyway, yeah MK has negative edge, which you may have already known and may not be surprised by it