Let's keep this brief. This game rocks. Here's why.
MK1 was sloppy and broken at first, and their attempts to balance it made it bland and boring. MK2 started out also broken, with tons of game breaking glitches. By the final revision, the game was solid, but boring. The people that played it regularly and with passion figured out the patterns and strategies necessary to win in high level play, and high level play was blech for MK2. Jax and Mileena ruled the game, with a couple of other characters able to maybe win with some luck and difference in ability. In the end, it was a battle to see who would move first and get punished for it.
Then MK3 came out. They decreased the amount of damage all moves do. They added damage protection to prevent juggled hits and autocombos from being as absolutely life-threatening as they were in MK2. They improved the general gameplay dynamic by adding the run button which turned the game into an aggressive battle. They improved the roster of moves by adding a lot more diversity.
Then UMK3 came out. It is so close to finishing up the game engine. So close to completing it. Added to the engine are jump punch and jump kick starters, which were loosely possible in MK3, but are far more solidified as a feature of the engine now. Characters like Kabal are balanced a little by reducing his combo damage. Characters like Sub Zero are balanced a lot (unbroken) by putting realistic limits on the ice clone.
In addition to this there are many many other gameplay tweaks, most of which take a lot of studying to spot.
What is the result?
UMK3 is the most competitive MK game to date. It is the most tournament worthy of all of them, and for me, the most entertaining fighting game around. The top tier is big, about 10 characters, and skilled players will use any character in the roster and be able to play them well. Compare this to the dominance of the top 5 in MK2 (mostly just the top 2), and even to completely different games like Capcom versus games where the top tiers can be a very small percentage of the total roster, and its a recipe for a fantastic game.
Keep an eye on the community, practice, get your skills ready, get into the tournaments. You won't be let down.