alt Hacker NewsEven simple Half-Life 1 mods built on textures and models from Half-Life 2 look much closer to 2 than one would expect. For example this mod, but not only:
https://moddb.com/mods/half-life-dark-future
You won't confuse it with modern Half-Life 2, but the original HL2 engine had far worse graphics than the latest version. Makes you realize how much of the difference between HL2 and HL1 is due to different textures and level design.
And Viktor Antonov (rip) art style.
edit: there is also the fact that map compilers for gold source games have advanced far beyond what they could do back in 1999. The lightmaps and light sources alone can be far more intricate nowadays than what you would get from the official valve ones in 1999.