Campaign has different difficulties but it's not like D2 where you progress throughout them, but you can only reach tier 3/4 after beating certain bosses.
End game is different for everyone. They have added plenty of stuff since I've stopped playing, but are constantly updating it and actually listening to the player base (I assume, youre gonna find squeaky wheels everywhere though)
But the campaign isn't really required either. If you havent played it yet, and arent broke, it's worth it. If you're struggling and living paycheck to paycheck, probably better things to spend your money on.