Injustice pretty much follows suit with the most recent Mortal Kombat. Adopting the familiar and effective combo system, adding in the infamous violence Netherrealm is known for, while introducing a new very fleshed out cast of DC superheroes and villains. 
The studio did a very good job with the character selection. Wandering away from their Mortal Kombat roots, it is very evident that everyone at Netherrealm are all big DC fans.
Aside from every hero and villain you expect in a fighting game (Batman, Superman, The Joker), Imjustice offers characters that I personally have wanted to play as in a fighting game forever such as The Flash, Deathstroke, and Bane.
Injustice also included heros and villains i didn't previously have any knowledge of. Which turned out to be a huge plus. Discovering new cool superhumans is always fun in the DC universe.
The 24 player cast Injustice gloats is probably one of the most impressive aspects of the game.
Mainly because of how thought through and unique each fighter feels while you're playing them. 
It's true, Injustice offers almost identical gameplay to the most recent Mortal Kombat, with very precise button combos and unique abilities to each fighter, but Injustice just had a better...I don't know... flow? 
Maybe it's because I love superheroes, but watching the Green Lantern generate green weapons from his ring during his normal combos is something that Mortal Kombat simply did not have. 

Environment also plays a pretty heavy part in Injustice. Not only is every map an awesome replica of your favorite DC cities and settings, but the use of TV's hanging from walls and random cars to slam on your opponent or maneuver around the battlefield is an all new feature that really mixes up what map you might want to choose and who you play as. 
Bigger characters such as Bane, for instance, may pick up a car in an attempt to crush his opponent with it. While more nimble characters such as Nightwing, will jump to the car and behind their opponent for a quick attacking advantage. 
Another huge difference is the introduction of wager clashes. Every so often your opponent or you can initiate a wager clash. Wagering sections of your "Super Meter" to either regain health or deal damage. 
The concept is really cool and the cinematics never really get old., but the computer annoyingly uses it at the end of every fight it is about to lose. Forcing me to save my Super and not preform my awesome abilities to win an annoying wager match that could cost me the fight. Not extremely annoying, but in a game that is nearly flawless it gets old.

Now on the off chance you bought Injustice for a story mode or campaign (not really advised) you may be pleasantly surprised. While injustice offers a perfectly cheesy, awesome superhero good vs. evil story. It only lasts about 5 hours and doesn't really show off the games coolest characters (IMO).
Of course as with any fighting game Injustice serves up a heaping amount of bonus content including game art for all the maps and characters and different skins that are unlockable throughout the game.
It also has S.T.A.R Labs, which is basically 10 challenges for each character earning you stars to progress to the next character. 
I understand certain people enjoy this, but for me it was a huge waste of time. I just don't really enjoy that kind of monotony. 
All in all Injustice was exactly what I wanted while giving me more than I expected. Nothing can compare to beating your friends as your favorite Superhero or villain. The fighting and controls are clean, smooth, precise, and rewarding. The characters are perfectly unique and the maps, while beautiful  are nice twist to a polished game. Injustice is a game that is just as fun to watch as it is to play.