There are 3 great things about Q2: its mods are superb, gameplay is just different than other quake games, and it has easily the most entertaining FFA in the series.

some of Q2's mods were way ahead of its time.
Rocket Arena 2 is the core of the most popular QL gametype, l-fire q2ctf is the most team-oriented ctf in the series and was a blast to play with air acceleration on, kots/kots2000/kots2007 is a concept that hasn't been emulated even to this day (and that could save QL btw), action quake 2's developers made counter strike (difference is aq2 is good), etc.
There are literally dozens of thousands of community-made content files, including mods, maps and paks. You just don't see that kind of community involvement in any game made afterward.

About gameplay, last month someone posted on ESR a makaveli interview, in which he said he just didn't like Q3's gameplay (mainly movement and not focusing on map control) and then dropped the game. So did lots of players, including thresh.
Ofc weapons are unbalanced, rockets are absurdly slow and so on. Thing is it is different than QW (netquake actually) and Q3, and that's great.


On the other hand, Q2 had some insufferable aspects, especially configuring it.
That's the only reason why I won't play it ever again since it feels very outdated on that side. I'd complaining about sens, accel, 125 fps (from what I remember clients back in the day showed that you could put 120 only and not exactly 125, which means screen tearing), lack of decently made picmip (it picmips skins), force models, lack of instant zoom and so on, all the time.
Maybe there's a way to properly configure Q2 in r1q2, but I know there's something unavoidable and that is what I hate the most about Q2: there's an input delay that persists even on LAN. It was removed in warsow; had something to do with how the game handles client prediction.

In the end the game is just outdated. Some people may think it's unplayable due to bad graphics, even with high res textures and using clients made in this millennium, and I can't play it because I'm picky.
Both things don't happen with QW (nquake), Q3 (cpma mod and QL) and Q4 (bearable).