Yes, it was lacking any story whatsoever, but from what I remember (it's been a while) the singleplayer levels were very well designed, fairly nonlinear and had tons of secrets.
(Still, the real meat was in the multiplayer, of course)
Quake 2 was all about multiplayer. Q3 dropped the pretense and essentially buried single play.
To be honest, Q2's "plot" was approximately 50x more detailed than Q1 or Doom. But expecting such an improvement with each new generation of FPS is asking a bit much.
I always considered it solid. Why?
It had a badass soundtrack, which I quite enjoyed: