I picked up a copy of Injustice: Gods Among Us recently, and played some single player while waiting for my wife to get home.
When she did, we launched a multiplayer game (both signing in with our Live accounts). She was really upset when she looked at my tag in the corner and saw 'level 11'.
She comes from a strong RPG background, and she went into the battle thinking she had no chance against me 'since I was so much higher in level'.
I managed to convince her, finally, that the displayed level was an account thing, which didn't affect my character's abilities, health, or damage. It may have been easier if I hadn't thoroughly destroyed her in the first few matches.
It did get me wondering what the level actually means. I know I get a few bits of in-game currency for unlocking costumes and music each time I gain a level, but is that all they do?