I just replayed the opening sequences of the original Half-Life. It's left me somewhat frustrated.
Despite the incredibly over-long sequence on the monorail, and the astonishing - and surely unnecessary - frequency of the loading points, it really still is the best introduction to any game, ever. From the moment you arrive you're being set up for what's to come, but without the game forcing you into rushing. You can explore the few rooms of the facility open to you, bother the scientists, explore the lockerrooms, blow up the contents of a microwave... It's you that chooses to trigger the beginning of events proper by stepping into the test chamber and triggering that resonance cascade. "They're waiting for you, Gordon... In the test chamberrrrrrrrr."
There's lots of time for silliness, like listening to the two scientists discussing that I'm a "highly paid professional", while I jump up and down on the spot. And then kaboom - it all goes wrong and the area you just familiarised yourself with has changed, been broken. And that's rewarding. As you try to find a way out, so much is happening. Bodies lie all over the floor, a scientist is trying to resuscitate another, people are panicing, and you're being introduced to how the enemies work - seeing the head crabs take their first victims.
Why has no other game managed this ten years later? TEN YEARS! It's ridiculous. It doesn't take much effort - you simple don't have much stuff happen for a while. You let the world feel real, before you fill it with space aliens and over-sized weapons. Developers, go back and spend half an hour with it, and for heavens sake, copy.