For me and Jalek this game was easy and we played on the hardest default difficulty. The only thing that gave us real trouble was fighting Super Saiyajin Reed. It wasn't even because I/We had Juggernaut, we breezed through the game for 98% of the game.
Thor and BP's voice were horribly done in comparison to their MUA counterparts. The voice of Black Panther is Josh from RE5 and he has that thick African accent that really kills the character.
The final battle was anti-climatic and pretty easy for us as well on Super Heroic. Just in case anyone is wondering, we didn't abuse anything, level grind or anything.
Iron Fist is one of the best new and unique characters to date. His gameplay was awesome. Songbird is good too along with Green Goblin being extremely unique. He can actually board his glider and shoot bullets.
The fusions were ok to me, some were great, others weren't like Steve and Reed together.
The best part about the game to me was the complete and utter ownage Steve handed Tony throughout both playthroughs. Just completely beat his ass that entire game.