How many players do it support at one time? And in what format? So is it like 2v2, 3v3, 4v4, etc.
3 to 5 in coop. The villain is played "by the game." Which is basically "Do anything that happens at the start of the villain turn" (cards will say what they do) then play a card off the top of the deck, then do "anything that happens at the end of the villain turn." There are virtually no choices involved. Eg. "At the end of the villain turn, this card does (H) projectile damage to the Hero Target with the most HP."
(H) being how the game scales for more/fewer players ((H) is "number of heroes that started the game"). So this would deal 3 damage in a 3 player game, 5 in a five player game, to one of the heroes themselves (hero targets are anything allied with the player that have hitpoints; the superheroes themselves will almost certainly have more current HP than any of their allies).
If you can't customize your "deck", what do you customize? Do you choose which combination of heroes/villians to take into battle?
There's a thing called "variants" which mostly effects villains, but I think a few heroes have it too. You use the same deck, but the character card changes slightly. That's it. You get your hero variations by playing different heroes (they're all modeled after comic superheroes, e.g. Legacy is "Captain America + Superman").
* Draco18s looks up America's Greatest Legacy, one of the Legacy variants.
"Unlock by defeating Ambuscade in Silver Gulch with no hero characters over 9 hp and not using Legacy on your team." (or clicking the "just unlock it" button, which bypasses--and does not award--any associated achievement)
Standard power is "everyone does +1 damage until Legacy's next turn."
Greatest Legacy's power is "One hero regains 1 hp and uses a power now."
What is the benefit of all the DLC characters/content? A lot of people are complaining about that aspect of the game. Would it still be fun even without the DLC?
It is still fun without the DLC, but at $18 I
highly recommend it. Each expansion adds more villains, more heroes, and more environments (environments vary between "helpful" and "harmful." After all, would you rather fight in Downtown where the police might show up and take pot shots, or on Mars where explosive decompression might hinder your ability to fight?).
As I said in the original suggestion, I was having fun with just the base set on my tablet and was only holding off on buying the DLC for when I got bored with what was available (my favorite hero character being Unity who comes in Mini Pack 1). Then it showed up in the Steam sale and I eagerly threw $18 at (I was willing to spend $30, just not right away).
By the way: not all combination of Heroes, Villain, and Environment are winnable! Some can be devastatingly impossible: like fighting the super intelligent AI called Omnitron
in the environment of Omnitron without any kind of AOE.