Half-Life the game change the idea of FPS games,with over then 50 awards and game of year award Half-Life present for us the meaning of smooth gameplay,horror with action tast.

Originally posted by Valve:

"Unlike many other games at the time, the player has almost complete uninterrupted control of Freeman, and the story is told mostly through scripted sequences seen through his eyes"

Half-Life supported mods the day its come out which open the way for modders to making great mods such Counter-Strike that become later the 1th online action shooter.
Half-Life was precedent the other games on that time with graphics,physics,animation,story and engine.
Worth mentioning that Half-Life multiplayer servers still full of players.