The balance is okay. It's just that Aquaman's ability is good no matter how the cards drop. My brief ranking of the DCCDBG superheroes, because why the **** not. Keep in mind that all cards are either Superpowers, Equipment, Heroes, or Villains and they have roughly equal distribution in the deck.
Top TierAquaman - you can place newly gained cards on top of your deck instead of your discard pile if they cost 5 power or less. Because it's type-neutral, you don't need to have a set buying strategy and can almost guarantee you'll always have at least two decent cards per hand.
Green Lantern - if you play at least three non-starter cards with different names during your turn, +3 power. Basically, once GL has a deck going on turn 3 or 4, you'll have an almost guaranteed +3 per turn. That's excellent. Type neutrality also helps greatly.
Martian Manhunter - if you play two or more Hero cards in one turn, +3 power; if you play two or more Villain cards in one turn, +3 power. This means you can focus on two card types and get +3 bonuses often. The bonus has a power ceiling, but that ceiling is a crazy-good +6 that Batman or Superman can rarely achieve. Only reason Green Lantern is better is that his bonus is more randomization-proof.
Mid TierBatman - each Equipment card gives +1 Power during your turn. Batman's is probably the simplest hero ability - buy as much equipment and equipment bonuses as you can. If a lot of Equipment pops up, he's awesome. If it doesn't, he isn't any better than other Superhero cards.
Superman - each different Superpower card gives +1 Power during your turn. Superman is almost the same as Batman, but because of the commonality of Kick they add that "each different" clause. Superman's still good, as Superpower cards are common and often excellent, but Batman is more repeat-friendly.
Flash - You always go first. Once per turn, whenever you draw a card, you may draw an additional card. Always going first is neat (but mostly forgettable), but extra draws are always welcome. If you get a Location card or some cheap draws in the early turns, Flash is very effective.
Low TierCyborg - after you play your first Equipment card in a turn gives +1 Power. After you play your first Superpower in a turn, draw a card. Like a gimped half-Batman half-Superman. Cyborg can focus on two card types for his buying strategy, but can't get a better bonus than +1 and drawing a card.
Wonder Woman - whenever you gain a Villain or Supervillain, you may draw one additional card at the end of your turn. Wonder Woman has a good ability, giving you additional cards for defending against player attacks OR for your next turn, but she has to buy villains first. Diana gets no card-specific bonuses, and as such has trouble building Power in the early game.
TL;DR Other people should play this game so I'm talking to an empty internet. New expansion is coming out in a few months (
boom). Sorry for being so unnecessarily verbose.