I’ve played through most of Episode 1 now (until I hit a bug that killed me and I lost the engagement to continue).
I like the leveldesign so far. The game is making use of many sharp turns behind which often a whole army of enemies is waiting. Stylistically it reminds me a lot of later games, including the different Quakes and later Dooms.
As others already mentioned, the level design forces you to pass through the same area multiple times, to get different keys or use levers that open doors or pathways.
Because the levels are relatively small overall, backtracking through already cleared areas is a lot less tedious than it would be in a level of modern dimensions.
There are often smart and less smart ways to approach a certain level. Often you can kill off a bunch of enemies from relative safety before entering a new area.
Two things I don’t like very much though:
a) When getting killed in a level, it is often _very_ hard to survive the first wave of enemies. You start with nothing but the pistol and often have to improvise, kill the first enemies (that don’t wait for you!) in a certain order to get a shotgun, run over radiated water multiple times just to survive the first minute of the level or so.
b) The lacking AI forced the level designers to replace smart enemies with hordes of enemies. While that I feel that is part of DOOM’s gameplay, it also feels older than the rest of the game from today’s perspective.