Well whaddaya know, Karolina Dean literally just got released in the Facebook game too. Whatcha waiting for Indignate.
I mostly hated the impossible grind of running Special Assignments in order to unlock characters. The nail in the coffin for me with that game was when they started locking missions and the only way to unlock them was to own multiple "premium" characters.
I've been playing the game since about half a year after its initial release in March 2012, so I haven't had any problems keeping up and recruiting all the characters for free. I can see why a newer player would have problems, however, since the games' first few Special Ops, they've been periodically releasing those time-based characters for that game's equivalent to splinters. So technically speaking, you can recruit (almost) everybody for free.
The only characters that, at this point, are locked behind the premium $ currency are playable villains. And you can earn that currency in the game as well (though nowhere near enough to buy all the available villains).
If you coordinate the hero currency with the time-based missions, you can recruit those heroes for free (not considering time of course).
That said, I can completely understand being put off by a huge 100+ character roster, and a lot of timed content. But that's how social freemium games work.