At the beginning of the game's life interactables were frowned upon because they were too strong and lopsided matches depending on the stage. If you won or lost could be greatly influenced by the stage picked. They have since been nerfed, a 5-5 won't become a 6-4 because of the stage selected and there are numerous universal and character specific tools to escape or even punish interactables so they can't be used mindlessly, but it does still have some influence on match-ups. That's what I don't like about them, I do like how there are resets and specific combos involving them. Characters have numerous stage specific combos, tricks, set-ups, and resets that can be used if you know your combos and can get to the place you want to be. Unfortunately, at this stage of the games life there are hardly any players doing it. It's a shame since aside from being used strategically they be pretty stylish, Batgirl's car bomb vortex was easily the most hype thing Injustice at Ev- oh wait! It was one of the only hype things there .
I personally like the the health system, no reset to neutral, just goes right back up. The mechanic I dislike is clash, due to being unable to stop or bait it albeit dropping your combos and zoning them to death. I cry inside every time I know that someone is about to clash, but all I can do is let it happen. It also breaks up the pace of the battle, favors zoners, and pretty much destroys comebacks.
By character powers to you mean how well they took the characters abilities from the comic books and translated them into interesting playstyles in game, or trait? I'm guessing trait. Giving a few examples, it ranges from:
Retarded
Aquaman: Let's him drop out of combos. Why the fuck is this in a fighting game? Most often used if you throw out unsafe shit and the opponent tries to punish. Thankfully has a longer recharge time now.
Doomsday: Gives him super armor for a few seconds. Let's your brain go AFK until it runs out. Luckily has a pretty long recharge time.
Great
Superman: Increased damage, breaks armor, can cancel from stings for combos, blockstrings, or tick grabs.
Black Adam: Recharges very quickly, let's you cancel normals, alot of chip, adds about 10% to combos.
Interesting
Zod: Summons a phantom beast behind the enemy. Allows for frame traps and mix-ups. Stops them from jumping since it tracks them.
Bane: For each level of venom he pops he gets armor on all his specials and some normals and increased damage. In return for making him godlike for a while, when it runs out he has a debuff period. He will become slower, do less damage, and take more damage. The extent of the effects for the buff and debuff depend on the level used.
Situational
Killer Frost: There is an extra "bar" for her. When fully charged all your attacks have more hitstun for a brief period of time, allowing for new combos.
Joker: A high and low parry. Whenever you get a successful parry joker receives a speed and agility buff.
Garbage
Harley Quinn: She takes 500 years to pick a random item that doesn't help in the fight at all. Use it if you want to insult someone's skill or you're trying to lose the match.
Cyborg: Stands in place and recharges health, very slowly I might add, with the assumption the the opponent is too busy masturbating to go over and stop you.