Recs.
Updated
SpecsUpdate
Pros
Pro Free to play
You have access to all levels and difficulties and the 4 starting heros. New heros can either be bought or unlocked by earning defender medals through mission completion and daily quests.
There are hero costumes which can't be earned in game but they are purely visual. The only advantage a paying player can buy is more bag space or extra hero card slots (you get +1 card slot when you unlock a hero with in game currency but can buy more if you want duplicates).
Cons
Con No challenge until the highest difficulty
There is a campaign mode but it's far too easy even for brand new players. And after you're done with the campaign, your fastest method of leveling to cap is running the same boss repeatedly on a low difficulty. Why have 8 difficulty levels if you encourage your players to skip half of them through poor balance?