Here is a good example of Casual vs Competitive and how it applies to games like this.
My friend and his brother are Casual Gamers. Now, he is a gamer. He plays to win. If he has free time, he will be gaming. However, he is casual in a sense that if a Game ever involves anything more than "Do X to complete Y" progression he will quickly lose interest. If he can jump in to a match of CoD or Halo where you simply point and shoot and get good at aiming to be successful he will play the hell out of it.
But, he is still Casual because he will never delve deeper into the meaningful parts of the game that require strategy and though and anything more than the face value of the game.
In fighting games (where this is often necessary or games like DotA or LoL) he will fall tragicly short.
So my Friend shows me MK9 for the first time. I had heard about it and was interested. Looked fun. I play at this house a few nights for a couple hours. He go back and forth do some easy 3hit strings and some specials. Win and lose. Throw in an X-Ray for good quick, easy fun.
I buy the game. I spend a few nights in the practice lab. I pick up Liu Kang. Learn some of his big hitting combos, learn some mix-ups, learn a few other things.
(this only takes a couple hours one or two nights to pick up. I honestly dont understand the "casual play for fun mindset" and i never will.)
We end up playing one night and im busting out combos left and right for huge damage. Juggling, mixups, the works. Well the friend's brothers instant reaction is "Wtf dude, you are being cheap! Stop doing those huge combos and just play the game. Pick someone else, thats a cheap character." I laugh it off because it is hilarious and its funny seeing him whine.
So i win a couple times, they pick someone like scorpion or subzero. Now, i'm not great at the game at this point so while i can do a few big combos and such, spam still throws me off as well as having to block High or low for certain strings. So they start sitting back spamming the ground fire as scorpion/his pull and the teleport to mix me up. And thats it. They spam 3 moves over and over throughout the fight to stop me from getting in. I dont complain. I know it's something i have to work on and i need to learn how to adapt to that playstyle.
So i lose a couple, but still win the majority becaus i have those big strings and better fundamentals. I keep getting better and they keep letting loose with their 3 or 4 specials they know how to do.
Well this goes on for a month or two. Then comes a time where i still want to play because im delving deeper in to the game and learning more and more and I know im not great with dealing with spam (and i know they can provide plenty of practice) and they have stopped learning. They dont want to play because it's boring and i usually win.
So they shelved the game and it's on to the next big thing for instant gratification and a feeling of success, whereas my gratification comes from learning and progressing, theirs comes from winning.
Once a casual player gets to a point in his career where he can no longer win the majority without having to learn anything new he will quit the game unless the game is so great/compelling that they are drawn to in. Few games these days have that.
How does this apply to Injustice?
Well its like mk9 all over again. Friends said they bought injustice and its awesome and i check it out. Like what i see. Buy it. Spend a few nights in the lab. Pickup some strings/combos. Learn a few things. Meet up. They had already returned their copy for a new games. Luckily i brought mine. We play, i do what i do. Tears/whining from the brother.
My friend grabs superman (im Grundy). He spams left and right with lasers to keep me out. When i get close he does the flying downward kick (have to block high) or the BF-2 or whatever punch knockback to get me back to range.
I lose about 10 matches in a row. Say wait a minute, do that attack on me real quick. I block, press a few buttons to see how i can punish it. Do the same with the other move.
Say okay, we start again. I change up my playstyle, i've learned how to punish his garbage. and i start winning a few rounds.
Well we've played about 15 rounds at this point and he is done. Says "i feel like im doing the same thing over and over and its kind of getting boring"
Meanwhile, i am having a great time actually learning the game and doing better. I want to keep going because it is awesome practice that i know ill need for later when getting in to online play vs good players and spammers alike. I could play those matches all night until i feel im competant enough to handle the situation. He is unwilling to adapt and to learn. He is a casual player.
TL;DR
Causual is a mindset. It has nothing to do with the game or the player. It has nothing to do with how hard it is or how easy it is. It has everything to do with how much you are willing to lose to get better. How much you are willing to learn to get better. A competitive players strives to be the best and will do what it takes to get there.
A casual player will play until he can no longer win enough games to satisfy himself and then be done with the game.
A competitive player will play until he no longer loses. (and then probably keep playing just to rub it in to the chumps that bow before him)