It's interesting how many people express some degree of ambivalence toward Quake 1 these days, it used to be considered unassailable in online discussions. I like the game a lot but I did always find the muddled visuals kind of dull, and the lack of any real plot (even a basic one like in, say, Doom or Blood) puts me off it a bit.
I was big into FPS games as a little kid but I can remember when Quake came out and my uncle had it on his computer, I played it for like ten minutes then just went back to Heretic. It took me a while to gather up the interest to play it in full, not really sure why. Doom's weird sci-fi and Heretic's dark fantasy and Blood's comedy-horror (and later, Unreal and Half-Life's spectacular and distinctive visuals) all grabbed me right away but Quake's dark brown mush was a lot harder to connect with.
I never really got what Romero was thinking; in the three years since Doom had come out, there'd been things like Heretic, Marathon (almost feels like a sci-fi novel condensed into a game), Hexen (blending in adventure game elements and expanding the rules of what an FPS game could be), Star Wars Dark Forces (basically a Star Wars film in videogame form), and even things like The Elder Scrolls. Tons of new possibilities for first person games, with developers constantly driving forward the limits of the engines and trying to design more expansive and believable worlds. And in this rich and exciting climate, Romero basically thinks they should try making full 3D Doom with less plot. The surreal Lovecraft stuff clearly works for some people but the levels have never really come across as nightmarish and eldritch to me; more just the result of people messing around with a newly-built 3D engine and thinking "ooh we can have staircases and underwater bits now".