Well I think Q4 feels a LOT like Quake 2. A whole lot. Same sterile gameplay, similar enviros, same bad guys, same weapons, same stupid AI, etc. Quake 4 is also equally as mind-numbingly linear. So much so I can't imagine wasting my time playing thru the game. You get an autopiloted squad now though. Yay. And I did make pretty significant progress into the SP.
I don't and didn't like Quake 2 much, although I have tried to get into it as recently as a year ago, and several times before that over the years.
Seriously Quake 2 is the same game as games that came before it. It didn't reinvent/invent shit for SP. If there's anything redeeming about it, it's its MP which apparently SOME people REALLY liked. I dunno. I was into Doom and Doom2 MP, and then played Unreal MP. With a little bit of Duke3D thrown in, but not significantly. I skipped Q1 & Q2 entirely during their peak years. Quake 3 was somewhat of a phenomenon, but at my college we played Unreal and UT way more.
Personally IMO Quake 3 is the only reason most people know Quake exists. I have never personally run into a Quake 2 or Quake/QW fan, although I always hear on forums how great it was. Quake 3 was and is big for many. I think the Quake 2 love is nostalgia honestly.