You know, I'm replaying HL2 right now and I understood one thing, modern gaming got so linear that I had troubles to navigate some of the maps in the Ravenholm, something I never had problems with before, and HL2 isn't exactly super non-linear itself, it just has a great hand crafted level design.
The other thing is, I don't think I've played AAA as diverse as HL2 ever since, the game constantly throws you into unique situations, it simply never gets to a point where it's a chore to play.
And the final thing, zombies wailing in pain is probably the scariest (and saddest) piece of sound design.
I wish Valve would remake it in Source 2, I don't think it needs much changes to the overall formula (maybe cut a boat part a bit) since it holds up damn well, but graphics obviously could use some refreshments.