There are minor differences in characters in MK when characters aren’t palette swaps. In the case of palette swaps the difference is the special moves. To break UMK3 down specifically:
Sweeps in some cases have different animation counts, sometimes before contact and sometimes after.
Kano, Sonya and Jax all have the exact same sweep duration for start up, contact, and recovery. Their recovery is faster than the other 20 characters in the game, which means, in a sweeping contest, they can do more sweeps in 10 seconds than any other character. he difference then between these 3 characters is the collision detection per sprite. A cold collision box for any sprite in MK is 100% the height the sprite, and approximately 25-30% the width, centered. So picture a rectangle running up the center of the character. So for Jax Sonya and Kano, their cold collisions depend on the height and width of the frames. Their hot box collision however is something more manually predetermined than that generalized 28% cold collision box width. Since there are no physical double hits in MK, a zero frame differential priority is randomly determined at the start of the round, ie: Sonya vs Kano, if Kano is chose, identical sweeps will always be won by Kano, however, other characters will always beat them by mere ticks in sweep start up even though they have faster overall sweeps than anyone. So, here we have the same sweep for three characters, and each has their own collision propertiers that are of course, subsequently related to their sprite size.
Similar to any fighting game, priority depends on the location of the hot box in relation to the cold box. So say a character has a very narrow sprite for an attack, their cold box is also very narrow, but the hot box might be very similar to say, a rooundhouse collision. A good example is Nightwolf’s Axe Uppercut. This move has a very narrow cold box on collision and a logically far away hot box since he’s holding a weapong. The move itself does 12% damage and activates damage protection, meaning every hit thereafter does half the normal damage. This move is excellent anti air and anti cross up because of it’s collision, but the damage you can attain off a single axe is relatively low vs most characters.
Roundhouses are big deal.
Liu Kang’s roundhouse is the best RH overall, with the male ninjas, Kung Lao and Kabal also being high up there. Collision detection for Liu’s RH is very different than say, Sub-zero’s whose anti air RH abilities are limited due to start up and his collision detection upon RH contact. Liu’s foot is a mile away from the center of his body and reasonably high plus he has normal start up time so his RH is godlike, however before someone assumes that something as simple as a RH can make an MK character good, his RH alone doesn’t put him anywhere near the top of the tier list. Sheeva’s roundhouse has a lot of range, but the start up is slow and can be snufed very easily.
Jump kicks all have different properties and some are easier to anti air than others. The best aa for a jump kick for damage is an aaHP, which can lead to additional enormous damage for Sindel, Ermac, Reptile, Stryker and Human Smoke. Big damage for Kabal, Robot Smoke and Cryax. As for the JKs themselves, Nightwolf and Kano have the best JK range in relationship to their cold collision in the game. They are very hard to anti air while in their contact frame, while someone like Sindel is very easy to hit even when she’s in her contact frame. For Kano and Nightwolf you have to wait until their attack duration has finished and by then they might be too close to the ground to get anything substantial against.
Run jab effectiveness has a lot to do with the pushing area of the sprites, the female ninjas have low locking run jabs, while some characters like Jax are higher up allowing for ducking LK’s to sneak in more easily.
Ducking LKs are important to fend off rush down as well, and Sonya has the best ducking LK in the game. Against even high level players her ducking LK strats are annoying to work around, but it’s not something to rely on. Kung Lao also has a very pointy ducking LK, but Kano on the other hand has a slower clunky ducking LK as does Liu Kang and Jax. Robot Ninjas’ and Sheeva’s D+LK is good, but not as good as Sonya or Kung’s and the female ninjas are somewhere between that. The differences may be small, but MK is about very small windows of timing.
One other thing I want to touch on, is the concept of spamming projectiles, as it relates to old school SF and MKII. No one can tell me that Guile Sonic booms in SFII are any different than Mileena sai spamming in the respect of, a good Guile player knows how to space time and zone those Sonic Booms that even a high level player has trouble with them, much like Mileena’s sai shots can be placed, in fact Mileena’s air Shot can have a lower place collision box than the ground one in MKII. These high players don’t wrecklessly use the fireballs just as high level Mileena doesn’t throw sais out when it’ll get her in trouble.