Putting The Player In Full Control
The first thing players were greeted by in Valve's debut title was a simple tram ride to work. As Gordon Freeman, a silent protagonist, you're on your way to partake in a critical experiment at the Black Mesa Research Facility.
This opening serves two functions; on an aesthetic level, it seamlessly introduces players to the setting they'll be fighting through, complete with robots, radioactive waste and scientists going about their daily business. Before it all goes to hell, you're given a window into how the place operates.
Secondly, the tram serves as its own tutorial, allowing players to get to grips with the controls. There's no forced training sequence that disrupts the pacing; instead you're planted into the game with its immersive qualities never breaking. There are no cutscenes, lines of text or setups to be found here. You see everything Gordon sees and this had never been done before in games of the time.
It wasn't just for the story however, as this affected gameplay as well.
With the resonance cascade plunging Black Mesa into utter chaos, only Gordon Freeman, encased inside the hazardous environment suit (HEV) has the power to put an end to it.
You'll begin the core action in earnest by making your way out of the test chambers, acquiring new weapons and engaging deadlier foes. How you proceed is often open to interpretation; players can make use of the environment and other NPCs as well as their arsenal to tackle obstacles.
This goes hand-in-hand with Gordon's own identity; is he a heroic scientist or a bloodthirsty psychopath? Benefits for the former including offering a health boost for those who save scientists. Whereas killing security officers will yield a greater ammo supply and even the chance to get weapons earlier in the game's campaign.
Add to this the vast variety of weapons and tools and you've got a single-player mode with tons of variation.