Lots of balance changes - main change is that difficulty now scales not just on numbers of enemies faced. Playing on Hell level could leave you fighting 100s of enemies - you will now fight a more reasonable number, but their stats will improve.
The same is true at easier levels. If the fight is against a single enemy, and the fight is judged too difficult, the enemies stats will be reduced somewhat.
Unlocking should now be fixed - hopefully, anyway, as I can only test this live in Steam!
(expect another fix in very short order if it is not)
The more you play, more artefacts and heroes will be unlocked. There is a screen showing your progress after every victory or defeat.