Quake 2 had better gunplay, more interesting enemies (yes, fuck you) and certainly better level design than any of those Serious Sam or Painkiller games. Quake 2 was a solid but not great old school game. Serious Sam was a cheap pretend old school game. There is no comparison here.
Quake II is an aggressively mediocre game. An apt comparison would be Daikatana or Blood 2. I'm serious, and I'm trying to hear a different opinion. I've replayed Quake II and Daikatana two years ago or so, and found them similar. Daikatana was even funnier, with the poor writing and all the broken stuff.
It suffers from bland weapons (bar the Grenade Launcher and the Railgun), bland enemies and adequate maps. The maps are sometimes competent, but the awful palette reinforces the "all is brown" sensation.
Gunplay? Quake II has a mediocre gunplay. Bar the Railgun, most weapons lack weight. In a shooter you can have gimmicky weaponry (DN3D Shrink Ray, as an example) or "effective yet unimaginative" guns (the Shotgun from Doom is the Ur-Example). Quake II has only boring stuff to use, and often not even that effective (Shotgun!). Not even the Nailgun gives us the "ooomph" that you expect.
Enemies? That's the weakest point. Blood II enemies are better designed and
used that Quake II enemies, and Blood II is an abomination. The Strogg have an uninspired graphic design (the biomechanical theme of Quake II is weak) with limited "game" design: the most used are mostly hitscan enemies/ranged explosive enemies. Lot of 'em in progression. The HtH/air foes are never used in a "smart" way (think of the first Skaarj in Unreal) or in sufficient numbers (Serious Sam). You find yourself gunning down 5-6 Guards or Guards equivalent with a Chaingun, and that keeps on and on and on in bland enviroments. The bosses are standard fare for its age: poor AI and bad maps cause them to be a breeze. Thus, boring. The foes lack "gimmicks" or peculiarities - methinks that after they designed the post-death attacks of the Guards and the Enforcer they said "Enough: too creative, guys."
Enviroments are weak both "thematically" and "mechanically" (we can all say that "immershun" is secondary in a good shooter, but Q2 gunplay is not that good, and have to look to brown textures 99% does not help). I mean, I'm trying to remember some highlight of Q2 levels, but only the timed secret level comes to my mind.
There is no reason to play-replay Quake II. It has nothing to teach, nothing to show. It is the 5/10 of its age. Daikatana at least
tried to do something great. Quake II is not "good". It's mediocre.
Of course, you're free to disagree. Some good counter arguments?