There is no way to summarise Half-Life 2 in an efficient manner. That's like trying to summarise the Moon Landings, or the Great Pyramids of Giza; there is just no way.

Half-Life 2, in my opinion, is the best game of all time. (so far)

Half-Life 2 executes all the problems it faces from a range to above average, to perfect. And above average is a stretch if you ask me. This game is larger than life, and for some reason, kept me enticed.

World Building and Overall Atmosphere.

Half-Life 2 has such a rich atmosphere. The old soviet architecture helps to give it this realistic vibe and kenopsia of what once was. It's the reason why Garry's Mod is so scary, and liminally active. It also helps, since the textures are made from real-life scans of images. It's nothing short of just perfect. An 11/10 atmosphere.

Story.

Half-Life 2 offers an amazing story. Hell, it's just so good. The way the Source Engine's Choreographer helps visually tell the story is great too. And the: "Show no tell" narrative is perfect. Valve even got the assistance of a psychologist to determine if the facial animations were actually human, and not just some robot trying to imitate reality. So, yeah, that's why Half-Life 2 is so much better than other facial animations in games. They just seem to miss the mark completely.
The gestures and animations are hand done, and are very realistic. Dr. Breen's conniving gestures are too well done actually. The Characters are, well, actual people. Not just bland, but funny as hell scientists. But, actual personality and judgement.

Graphics.

The Graphics are great, much like the textures. The lighting (and the HDR later on) are amazing. I like how the shadows look. (Although CS:GO and other updated Source Engine games have better shadows) The best examples of this are in Water Hazard, Ravenholm, and The Coast Section. Actually, the entire game; is a great example of lighting in general. I believe that HL2 still holds up.

Game-play.

This one is a woozy. I think that HL2's Game-play, for me at least, is perfect. The Physics Engine is so cool. Seeing a Cinder Block fly through a window and into a guys face is funny. The Gun-Play is great. However, other people have a variety of criticisms towards HL2's Gun-Play. 1. It's so easy, the AI is dumb. 2. The Guns sound bad. etc.

None of that matters though, because it's not for everyone. HL2 has very casual game play. I cannot say why this is, but i'm thankful that i don't have to earn the power trip like you do in the first game. It's kind of like a reward. Or a canonised version is that G-Man makes you immortal or something :D I love the game-play of HL2.

Audio.

As a man of composing myself, i love the sound design of HL2. It's very eargasmic. It feels very nice to listen to.
The Music is 10/10. Ambience with Electronic Drones and Industrial Guitars, and Drums. It's just too good.

Conclusion.

Whilst i think HL2 is the perfect game. HL2 can get boring when re-playing it, and there are some minor bugs. (Lighting Bug, and some wonky physics.) Some Chapters stretch out TOO much. (Water Hazard has great atmosphere and lighting, but damn, can get boring) I would give HL2 a 11/10 solely on my own opinion. May this game live on forever, as an amazing piece of art, and game design. Everyone should play this game.