That's just it; if you weren't there when it was new then you've probably already played numerous solid shooters and story-driven FPS games made since HL2. I'm sure it was revolutionary at the time but, nostalgia aside, it's a whole new world out there for the genre. I mean people talk about Ravenholme but I mainly just remember tapping monsters on the head with my crowbar, tossing crap around with the gravity gun and that platforming you were required to do. I don't remember it being a great level though or especially atmospheric. On the other hand, my first visit -- hell, my third visit -- to Lab X-18 in Shadow of Chernobyl was far more nerve wracking than anything I came across in HL2. Now, to be fair, SoC came out well after HL2 but that's sort of my point.
STALKER: SoC felt like a drastic improvement on what Boiling Point: Road to Hell tried to do w/ the open-world FPS. STALKER: SoC had more emphasis on FPS, a quite bit less on RPG stuff. Boiling Point (aka Xenus) also tried to infuse some RPG elements in there (i.e. you upgrade your skills by practicing them like Elder Scrolls games) - but like some games from Ukraine in that era (2005-2010) that GSC Game World (STALKER series) + Deep Shadows (Boiling Point: Road To Hell + its sequel White Gold: War In Paradise; and The Precursors) popped-out namely, it was very ambitious in what it set-out & tried to do. STALKER: SoC was quite buggy upon release, but nowhere as buggy as any of the games from Deep Shadows.
Sure, I would expect STALKER to be more horrific than HL2. STALKER mixed elements of survival-horror, open-world games, and action. A lot of STALKER's tension comes from its setting, the lighting, monsters, anomalies, and its difficulty. At the default, the game's just tough as nails b/c the AI wouldn't miss from miles away and when you shoot someone - yes, even humans - in the head it wouldn't always down-right kill them. There were other problems for the player, where your gun can jam and you have to react quick to hopefully unjam it or switch to another gun; maybe be foolish and run into an anomaly (you should've checked and tossed a bolt ahead of time to check for that!); or run into some other kind of monster that bites you + scratches you so you have to use bandages to stop the bleeding quickly before you bleed to death (if you even still had bandages on you!). The game just basically tried to find any way it could to cause tension and punish you.
HL2 was never really a horror-game, though - it was basically a linear, story-based + character-based FPS/action-game. HL2 just never ever struck me as scary ever, PERIOD. A lot of its linear levels had some kind of gimmick + theme to it - whether using a vehicle in a level to move around + run over ant-lions; use the regular gravity gun to grab objects and toss them around; use the human-tossing gravity gun to toss humans around; physics puzzles; and/or Ravenholm where you fought zombie-like enemies where you had to use the environment to your advantage in every way possible. HL2 when it came out absolutely set the bar for FPS's with its variety of level-types, gimmicks in levels, and whatnot for a linear-shooter. Many tried to copy HL2's formula (like say FEAR 2 + TimeShift) - but they just weren't even close to being on HL2's level b/c....well, they basically tried to copy HL2's formula and didn't really do enough different on their own to separate themselves from HL2's greatness.