The first thing I think of whenever I replay this game is, 'They don't make games like this anymore'. I'm utterly shocked by the level design and game play experience each and every single time I play this. The pacing is as good as any game can be. Im not sure if any game will ever surpass Half Life's story line with out the main character saying one word throughout its entirety!
I dont think any game has ever immersed you as much as HL. Untill Xen, you are not really playing any significant part in the story. It seems like the Military and that creepy guy in the suit that keeps a tab on you are running the show. Opposing Force and Blue Shift only enhance the quality of the first one in this aspect. The Complexity of the Simplicity (yes I said that) of this game particularly stands out at me. A survival game has never quite captured me in as much as Half Life has, and still is.