Here is a list of currently known boosts and which cards activate them. The boosts are strange and I'm not sure about the exact percentages different team compositions offer. Some will trigger only when on defense and some trigger only on offense. Generally if it's a Hit to offense or a boost to defense, it only occurs on defense. If it's a boost to offense or a hit to defense, it only occurs on offense. Information for these was found on about five different forums going through post by post and I haven't verified them all.
One interesting thing to note, combos only seem to activate when the cards are either hero or villain. There appear to be no activations that are a hero and a villain combined. For example Wolverine and Sabertooth you would think would have a combo, same with Kingpin and Daredevil.
Five Card Combos-5 Heroes on Defense - Marvel Hero Team - Def Boost to Your Deck
-5 Villains on Offense - Dark Powers - Def Hit to Opponents Deck
-5 Male Heroes - Fury Blast - Atk Boost to Your Deck
-5 Female Heroes - Alluring Heroes - Def Hit to Opponents Villains
-5 Male Villains - Wild Villains - Atk Boost to Your Deck
Four Card Combos-Mr. Fantastic, Invisible Woman, Human Torch, The Thing - Fantastic 4 - Atk Hit to Opponents Deck
-Human Torch, Ghostrider, Thor, Storm - Ignition Spark - Atk Boost to those four
Three Card Combos-Daredevil, Captain America, Iron Fist - Combat Masters - Atk Boost to Tactics
-Wolverine, Ghost Rider, Human Torch - Bikers Organization - Atk Boost to those three cards
-The Thing, Hulk, She-Hulk - Unlimited Power - Atk Boost to Bruiser
-Mr. Fantastic, Iron Man, Dr. Strange - The Great Brains - Atk Boost to Your Deck
-Captain America, Iron Man, Thor - The Big Three - Atk Boost to those three cards
-Silver Surfer, Nova, Human Torch - Shooting Star - Atk Boost to Speed
-Ultron, Iron Monger, Sentinel - Modern Weapons - Def Hit to Opponents Heroes
-Mystique, Enchantress, Medusa - Coven Banquet - Def Hit to Opponents Deck
-Spider-Man, Blade, Ghost Rider - Ogre, Devil, and Spider - Atk Boost to All
Two Card Combos-Wolverine, Captain America - Super Soldier - Atk Boost to those two cards
-Wolverine, Daredevil - Brotherhood - Atk Boost to those two cards
-Wolverine, Punisher - Atk Boost to those two cards
-Wolverine, Mystique - Merciless Heroes - Atk Boost on those two cards
-Iron Man, War Machine - Drive Reactors - Atk Boost on those two cards
Please list any I haven't mentioned and I will add them.
Combo's I would think would do something but haven't found or been able to test:
Most Villain Combinations
Cyclops+Wolverine+Storm (dunno if any other X-Men are active)
Captain America, Thor, Hulk, Iron Man, Hawkeye/Blackwidow/Nick Fury (the movie and all)
Any combos involving Spiderman really, I think he should have more.