Easily one of the best games of all time.
Story: I was totally blown by the story. I was, at first, confused by the fact that the original story line, in half-life 1, would be changed in Half-Life 2. However, things connected, piece by piece, and eventually the story sucked me in. I couldn't stop playing until I find out what happens in a certain event and then be awed by it when I found out. The story was refined and definitely worked on, with a strong narrative. The game really let's you be your character since it stayed in first-person through-out the game much like its predecessor.
Graphics: The visuals of the game are awesome at the time. Not the best, mind you, but it's one of the better ones. The parts that are really important were given much detail. Valve put much work in making the environment look real. From buildings to walls, everything is simply great.
Game play: One word: Awesome. In the early stages of FPS, before HL1, the genre suffered from poor game play. You shoot then you finish it. But in Half-life, game play is clearly refined. Puzzles were entertaining, not a walk-in-the-park nor teeth-gritting. Weapons were given in just the right moment, meaning it has a feeling of reward in getting one. The gravity gun is a unique weapon, it is revolutionary in its day. It introduced interactive game play, the first in its genre.
In conclusion, HL2 really lives up to its name as the greatest example of a story-driven FPS.