A story without interruptions
Scientist Gordon Freeman causes an accident at the Black Mesa facility that opens a gate to another world. The player never loses control of the character and everything happens around him in real time. In 1998 that was a revolution.
Mods
Valve released tools for modifying the game and the community used them to the full. Counter-Strike, Team Fortress Classic and Day of Defeat were born. Valve hired some of the creators and turned mods into games of its own.
Half-Life 2 and Steam
Half-Life 2 in 2004 introduced the Source engine with physics (the gravity gun) and was the first big game that required Steam. Half-Life: Alyx (2020) is still considered the best game for VR.