I love how the games have the consistent goal of making you feel as if you're in your own action movie. What makes this work for me is the perfect choices they make in where to put music. All the games have these memorable moments in particular that stand out because of the awesome action music that starts at just the right time. In HL1 it was your first firefight with the soldiers. In HL2 it was the beach assault on Nova Prospekt right after you gain control of the antlions. In Ep1 it was when you and Alyx mow down zombies with shotguns in the hospital. In Ep2 it's the final part of the turret vs. antlion battle when the Vortigaunts show up. I love the more traditional scores from many games such as Elder Scrolls and Halo, but Half-Life's sparingly placed techno beats elevate those moments to true action hero awesomeness.
The various sounds are part of the experience. When I heard the suit's Geiger counter go off in this latest episode, a big grin appeared on my face. The weapon sounds are instantly recognizable, as are many of the voice actors and other sound effects. The Halo games have many familiar sounds throughout, but they don't give me that warm fuzzy feeling that Half-Life does. I don't know if that was intentional or not, but it works wonderfully in my case.
So overall I guess a big part of what makes Half-Life such a favorite of mine is the audio experience. You can tell that they've spent a lot of time on that portion of the game.
The other part of what makes these games amazing are the characters. Valve has spent a considerable amount of time on the AI and character animations for the people who accompany you through the Half-Life 2 saga. They also have amazing voice actors to give life to these characters. The original Half-Life had pretty good enemy AI for the time, but nobody accompanied you throughout the game. Through all of the Half-Life 2 games, there are recurring characters that you grow to care about. In Episode 1 especially, the character of Alyx is the best AI companion I've ever had in a game. She's written very well, and is genuinely helpful when battle hordes of zombies and soldiers. After a rather horrifying experience, she stops and almost breaks down. It made want to give her a hug and tell her everything will be fine. No other game has made me emotionally invested in my AI companions. The other characters are also interesting, and while not all of them are likable, they're all memorable. Episode 2 has at least two gut wrenching scenes involving these characters.
Anybody who calls themselves a gamer needs to play these games.