There are no unlocks. There are no achievements, challenges, story missions, nothing. The ONLY thing you get is XP which is towards levels. There is no level cap and every time you level up you get a treasure box thingy which has cosmetic stuff in it like skins, voices, that kind of shit. Doesn't affect game play at all.
You can't create classes, you literally join a match, pick one of the 21 characters and kill shit. Apart from cosmetic stuff, you can't modify the characters in any way so the playing field is always even.
DLC will be free, always. This will contain maps, characters and maybe game modes.
There are however microtransactions to buy those treasure box thingies but that's all fluff and doesn't affect game play at all.
For abilities, you have primary fire, secondary fire, one to three extra abilities which are on cool downs and a super. These are completely different for each character so you're always coming up against different compositions. There are healers, tanks, support and DPS.
There are three game modes: escort payload, king of the hill, and attack / defend. They're all epic fun.