Alright so I've played Hexen and Heretic for the first time (aside from years and years ago where I played a demo of hexen and maybe the shareware version of Heretic very briefly and wasn't that impressed with either) and ... dang I was wrong about them.
Heretic is straight up just The Catacomb Abyss but Doomified, so it's understandable I was a bit "meh" about it, but the monster designs are p. cool, and there's some neat touches here and there, environment wise.
Hexen on the other hand? NOW I get why it was so well received. It's a totally different beast to Doom. It's not "Doom but fantasy", it's its own thing, just happening to use a very modified Doom engine. There's far more of a focus on traps and puzzles, there's some absolutely wild detail gone into the levels and atmospherics, an inventory system of sorts, aand using items becomes rather important too for bolstering your attacks.
It's really good stuff, I'm impressed.