What had actually happened was a level change like that in Quake, but it was secretly hidden. The textures and sounds of the game really do deserve a mention. It's this move that has no doubt helped Half-Life look so beautiful, yet so varied and creative. They're so real. And with the addition of a complete sound system like A3D, the game is totally immersive, and very scary when playing late at night, volume turned up, in a dark room on a 21inch monitor.
Aliens sound like aliens, and reloading sounds like a reload.
One amazing advancement over Quake II is the way that sounds work in the game. If you're in a confined space then you hear echoes of weapons reloading as if you were in a confined space. Depending on where you are and what is around you, sounds change. You'll notice how I haven't mentioned your opposition. Now, while I could ramble on about all the enemies you'll meet, that would ruin the surprise for when you do buy the game.