A lot of reviewers have criticized this game for not having a lot of characters. I think there's some truth to that, but not a whole lot. Teenage Mutant Ninja Turtles: Smash Up does have fewer characters initially, but there are at least 10 secret characters. Some of these have been revealed online, but a few haven't (if you look at the game website, you'll see at least 6 question marks for additional secret characters). I have no idea if these secret characters are going to be exciting additions, like Leatherhead but it's too early to tell.
Also, let's remember that Super Smash Bros. Brawl had more characters, but less than half were even decent and most were lousy. Whenever I played with my friends, we tended to only use about 3 characters each. Frankly, adding a bunch of Jigglypuffs to the character roster did not really do much for me. By contrast, with Teenage Mutant Ninja Turtles: Smash Up, I get the sense that most of the characters are roughly equivalent and good to play with.
This game also has a few innovations not seen in Super Smash Bros. Brawl. I like the stages a lot better. More room to fight, as well as more interaction between the stage and the characters. In one stage, a giant alligator will eat your character if you're not careful. Also, the characters can hang onto walls.