it's the crowbar! that's whats so great in half life.
the hype goes all the way back to half life actually, it was the first game with scripted sequences replacing cut scenes, which added to the immersion and realism, and made playing the game a memorable experience. like everyone who played remembers the black out knock in the middle of the game. you answered the q yourself, its a "classical" game.
I still cant believe they made such an awesome gameplay with real scary environments in 1998. Ah, Gordon freeman, black mesa research facility. Takes me back to my childhood. The game was ahead of its time and the success unprecedented. While the later stages were not so exciting, the gameplay was taut throughout. I havent played HL2 till now , will have to that soon. Also finished HL Blue shift and opposing force. Opposing force was the best in gameplay among all HL expansion packs