I'm not going to go through everything that great about HL2 because then I might as well just send you a 30page essay, so here are some brief examples.
Story & characters:
Ex the first 30 minutes of gameplay. Shows you where you are and a quite a lot of what's going on. Establish a clear threat and show its dominance by various things like the huge walkers, fascist scary soldiers, tower of doom, creepy 1984 video surveillance, "friendly" Dr breen etc. You see all this and how you and the people around you are oppressed which gives you a good reason to care about shit. You encounter a couple of characters, old friends and soon to be new ones who provides help and gives you a sense of direction so you can head off towards the next encounter and start the adventure. All this without some game breaking cutscene or through some shitty exposition by 10-20-30 minute dialog or by a huge wall of text. Show don't tell.
AI & Enemies:
Combines vs Rebels vs Antlions vs Zombies = interesting shit can and will happen.
Weapons:
Gravity Gun trick #24: GG + Manhack = Zombie head slasher
Gives you a laser guided rocket-launcher that YOU control = 100x better than any autoaim crap you have in a "realistic" war shooter.
Alt fire on Pulse Rifle = Hilarius shit can occur.
Physics:
Still one of the best in the industry, feels, reacts and sounds right. Just the feel of shooting a barrel with the SMG can be very satisfying. Also it's used everywhere in the game like the vehicles, ragdolls, puzzles and of course as a weapon.
Graphics:
Technically not so great by today's standards maybe but still often much better looking anyway since you can most of the time
see what the fuck is going on.
Ex this:
vs this shit:
...or this boring brown mishmash:
...or this messy crap:
...or this... I don't even...
Music & Sound:
Fucking excellent, everything from the voice acting to the little stuff like when you hit the crowbar on steel barrel or the hear an Antlion come flying. When music start you always get pumped up since you know shit is going down but you never sure exactly how since HL2 has so many different well designed enemies you can encounter in all sorts of various combinations in various interesting and different environments. Speaking of which....
Levels & environments:
Everyone who played HL2 will easily remember the many of the iconic images you got from it, like the big tower and the feel of city 17, the horror in ravenholm, the mixed beauty of water and radiation in the hectic canals etc. Who the hell remembers any chapter from Halo 3, BF3, Killzone 2, Medal of Honor, Rainbow Six Vegas, Black Ops or pretty much any typical modern fps game campaign without perhaps thinking about it for a good while or looking it up in a play-through video?
.