The first trailer for Marvel / Gazillion / Amazing Society-developed MMO Super Hero Squad is up and it definitely gives the impression of the core gaming mechanic being card-based. This makes sense given that every player will want an established IP character, which involves a lot of different Wolverines – everyone can have a Wolverine card in their team deck if they want to.
Wizard101 comes to mind as another MMO that uses card-based gameplay and that is a pretty fun title. Ultimately it all comes down to how the card game is played that will determine how fun SHSO is, but it is an interesting starting point.
World Class Villains 
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