Alot of peoples love for HL2 is very much rooted in the 'you had to be there at the time' mentality. When that game came out it was doing a tonne of crazy awesome stuff no other game had before but naturally with time those elements have become less unique and less impressive. This was a game that released at a point where people were still being blown away by the fact you could walk up to and interact with characters in first person and they would actually animate and emote believably.
Imagine that Uncharted, Assassins Creed, Killzone, Gears of War, Halo and any number of other current gen big budget impressive games didn't exist and the closest thing to them was HL2, that is what that game was, it was a demonstration of the future of story development, character realization and immersion evolution in the gaming medium, it was a fucking masterpiece of its time. All of your favorite modern games have it to thank for showing so many studios 'this is how you do this awesome shit well' and Valve continues to do that to this day with the likes of Left4Dead, Portal and so on.
Everything from Dead Space to Bioshock, HL2 in some way had an impact on them just as Doom, Mario, Zelda and other classic games of the past changed the industry forever.
To not understand why HL2 mattered and why it was amazing for its time is to fail as a gamer, to judge it now based on modern standards is unfair at best and utterly moronic at worst. Yeah its kinda slow paced now and the environments are sparce, the enemies simple, the puzzles easy and the graphics pretty dated, why? BECAUSE ITS FUCKING OLD NOW.