Why not keep the names "Normal/Nightmare/Hell"?
These are iconic to the series, and you're just getting rid of them for no good reason.
Let me try to anticipate the answer: "Players might get confused".
The main reason is that the tiered Normal > Nightmare > Hell > Inferno system is gone completely; the new system works quite differently and we didn't want the names, which have a history and thus an expected way of working, to make players think the system functions one way (when it in fact functions in a whole 'nother way).
Totally fair feedback, though. Happy to pass it on.
They might be confused by it anyways regardless due to the names being changed and it being a new system, so its kinda doing the same thing there. The only difference is in name only and it isn't hard to change the names. You will just add the "you can unlock this by getting to the required level" to the difficulty mode type. I would say that is self explanatory.