What's there to expound on? Half-Life and Quake 2 are both 3D games with not entirely dissimilar weapons and a variety of enemies, except Half-Life's level design is railroaded.
It's not like Q2 is the epitome of nonlinearity either (though at least in terms of spatial layout it's levels are nice - especially the palace hub where you can bypass somewhere around 60% of upper palace, thematically they are monotonous and suck, though), plus Q2 has an important disadvantage:
The part of this shooter where you shoot shit and avoid getting shot yourself - AKA the shooter part - sucks horribly.
Pretty much every enemy in Q2 has some crippling delay introduced before seeing you and firing.
Pretty much every enemy is not only incapable of leading the target but incapable of keeping even a hitscan weapon trained on moving target with something like 0.2s tracking delay. Some can't even track at all once they start firing.
Enemies, even those using the same types of weapon as you do, deal about half of damage and that's on hard (it's halved again on easy IIRC), which means that even if you strip naked and stand in front of fucking gladiator with a fucking railgun waving your dick in his face he won't kill you with one shot and will barely manage to kill you with two - provided you stand still allowing him to hit you. In comparison, gunner, which is supposed to be armour-plated cyborg monstrosity is not just killed but gibbed in two shots from *your* railgun. One if you are stealthy and get 2x backstab multiplier.
Pretty much every enemy has lengthy, easily triggered pain frames meaning that you will likely be able to prevent them from firing at all even with a slow weapon.
No enemy save for brains (which is completely harmless due to moving so slow that snail could circlestrafe it), has any sort of armour or resistance to different types of damage.
Barely any enemy can navigate shit.
Then you have pretty nice armour with pretty nice damage reduction mechanics that would actually work nicely in some other game where you can get hit hard, power shield you don't have to ever deactivate after first finding it, and tons of powerups you won't even manage to use up before finding replacements.
That's in addition to ample geometry to hide behind, that ensures whatever protections you have will remain mostly unused.
And then you have fucking chaingun to ensure that even the toughest enemy won't last more than several seconds if for some reason you don't have anything to hide behind - shit'll fucking gib a tank before it manages to fall down if you don't let off the trigger fast enough.
You can quad it too for even more excessive rape.
The only "advantage" of Half-Life over earlier FPS models is its "storyfagness" and the fact that it was the first FPS where people's mouths weren't painted-on textures.
Q2 is strongly storyfag, you have clearly defined and arguably sensible train of objectives you pursue in clearly defined succession of locales, the difference is that it sucks on this front too due to being full of monotonous poo.
As much of a storyfag I am, however, I've yet to be sucked into a story in any FPS game.
It's not as much story as sequence of events, enemies and locations making some sort of sense and staying diverse to avoid boredom. Q2 utterly fails at the "diverse" part and doesn't exactly pioneer the "sense" part (and is very simple in this regard too, even compared to much older games).
In order to observe the difference between the design of earlier FPS and Half-Life, one should observe the difference between 1996 version of Tomb Raider and its current "reimagining".
In the 1996 version, you immediately started off in the game, somersaulting and shooting your way through the levels.
In the current "reimagining", the game starts with you basically pressing forward over and over as your avatar stumbles between EPIC SHIT happening that you have zero control over.
The more such sequences there are in the game, the less replayability it has.
Indeed, but HL doesn't have many such sequences - intro is railroaded, obviously, but after that, it's not really worse than Q2, with possible exception of (aptly named) "On A Rail" chapter where you spend most of the time riding a rail car while shooting shit and negotiating hazards.
Ok, "Residue Processing" was pretty abysmal platformer section.
I've never desired to replay Half-Life, simply because there's no way to speed up the intro sequence
There is and a very simple one - make save at the end of intro part, keep it.
Half-Life didn't even manage the same level of narrative all the way through. It was front-loaded, mediocre in the middle, and ended with possibly some of the shittiest level design of any FPS, known as "Xen".
At least Xen made an interesting and alien environment completely different from everything you've seen during the remainder of the game which alone puts it ahead of Q2.
