I enjoyed HL2 back when it was new, mostly because my teenage self was impressed by the graphics and physics - and, to be honest, those are still pretty good and I actually really like the look of the Source engine in general - and because I was still comparatively new to FPS, so the genre itself still fascinated me.
But when I play it now - meh. It's just not that great, especially compared to contemporary or earlier games like Return to Castle Wolfenstein or the Jedi Knight series, which had such insanely better level design it's ridiculous. HL2 focuses too much on physics puzzles, vehicle sections, and fighting together with teammates like it's Call of Duty, and neglects the parts of an FPS that are actually important for an FPS to be fun: gunplay and level design. Most of the game's levels are decidedly unimpressive. They have little verticality, and are mostly linear - the open segments are usually filled with goddamn fucking vehicle sections, and vehicle sections suck in any FPS ever, because they're basically just a gimmick.
HL2 is a game of gimmicks. Gunplay and gun variety are shit, compare that with RtCW that has versatile standard WW2 weapons like pistol, SMG, long-range rifle, silenced SMG, the FG42 which serves as an assault rifle, oh wait I didn't even mention the Venom minigun and that electric gun yet and it's already a more diverse weapon set than what HL2 has to offer.
All in all, HL2 is like one of those mediocre early 00's shooters of which there were many. Solid entertainment for a few hours, but too many meh parts to make a replay worthwile. I'd rather replay RtCW for the seventh time, or Unreal, or the Jedi Knight series, because those are all better games. HL2 is the better tech demo though, that's the one thing it does right.