In terms of story mode, the quickest way to demonstrate what's wrong is to do a comparison of the first chapter to something like the Netherrealm games.
Here's how it goes in something like Mortal Kombat or Injustice - you get an opening cutscene to set the stage, and then you're given control of a character, usually a well-known character, and you fight for several standard fights (normally three). Your opponents will be a selection of the other characters in the game, sometimes with special conditions applied. You'll usually have short cutscenes to briefly explain why you're fighting those particular characters. After you defeat them you play against a boss/rival character and then you swap to playing as someone else. You spend long enough with each character to learn at least the basics, but not long enough for them to get boring if it turns out you hate them.
Now let's look at how MvCI opens its story.
It's 88 days since the convergence. It's probably pretty obvious to MvC fans what that is, but this is not explained. The game opens in media res. The characters all know each other so if you don't know any of them, tough shit. You do start off with a relatively iconic team (Captain America and Mega Man X), but you're against generic Ultron bots. You smash them to bits in seconds.
Then it plays a cutscene showing Chun Li and Captain Marvel smashing more Ultron bots. More Ultron bots arrive in a cutscene and get smashed as well. Despite the game clearly establishing these bots as zero threat, the next fight is just you, as Chun Li and Captain Marvel, against more Ultron bots.
Next up, Thor and Arthur vs Ultron bots, then Rocket and Dante vs Ultron bots. The cutscenes between these fights make it clear that the characters have a plan, but you don't have a clue what it is. They're doing the stupid thing where they talk around their objective to avoid spoiling things for the player, but that just makes it difficult to care what's going on.
After that, Thor and Mega Man X against "Xgardians". It's not too difficult to figure out what these are, but the game doesn't bother to stop and tell you. There's another fight against Ultron bots and Xgardians, and then you fight Ultron Sigma who has cheats on and who will stomp you, and then you head back to Avengers Tower for some exposition.
So quick recap - in six fights (which probably last for a minute of gameplay time each), you play as I think nine characters, against two tutorial characters. You haven't had time to learn how to play as anyone. Unless you like a wide swathe of the roster, you've probably been forced into several team-ups with at least one character you dislike. You've only fought against one named character and he's destroyed you. You have no idea what's happening with the storyline - you probably guessed what the Convergence is but even if so, the game's opening scene tells you that you've missed 88 days worth of plot developments, and there's almost no effort made to start filling in that gap.
It's a mess, from scene-setting to plot progression to how it ties into gameplay.
