I played Morrowind when it came out. I still remember getting super-engrossed in the alchemy subsystem. (Unfortunately I also eventually figured out that a positive feedback loop in the alchemy subsystem and the way that some shops have an infinite supply of goods resulted in a way to make my character ludicrously overpowered, at which point the fun leaked out of the game for me). The sense of exploration and discovery I had when I first started exploring the alchemy system in Morrowind is something I've wanted to try to recreate in a tabletop RPG design.
OpenMW fixes some of the weirder things with the game mechanics, though alchemy is still pretty powerful because multiple effects stack simultaneously.
Morrowind's real claim to fame is just how many mechanics and systems it crams in without getting too weird and incomprehensible. It's not easy, by any sense of the word, but it does manage to be comprehensible, which is important.