Personally I think that your idea of class leveling is dumb. Everyone should have access to subclasses regardless of what difficulty they play on! The difficulty one chooses is up to the player, and to have such a major factor of the game hidden from people who don't play on harder difficulties is just absurd. Subclasses unlock by level always (sometimes story completion, yes), but not by some difficulty setting, that is only supposed to make you level and reach subclasses faster. They are SUBclasses after all, so it's inherit that, regardless of difficulty, people be able to attain them. One last thing, "should we as gamers unlock them through hours of gameplay, death after death because the reward will be great at the end!?" Yes, we should unlock them, they are indeed SUB classes, meaning alternates for people who level to 15 (death after death is also pushing it... There's not even a penalty for death).

I suspect you're referring to the subclasses. Each class has two subclasses (maybe more in the full game). You unlock the second subclass at lvl 15. That would make for 6 subclasses total across the three main classes. The subclasses you have equipped determines your Super, Grenade, melee, and passive abilities. For example the starting Titan subclass has a ground pound as it's super, and the subclass at lvl 15 deploys a bubble shield to protect you and friendly players within from damage.