Half-life did a few things really well that increased immersion. Some of the easily identifiable ones are the HUD, which is completely non-invasive, and never removes the player from the experience. Compare that to Bioshock, which had a rather clunky HUD, and you will see what I mean. At no point did you leave Gordon, visally or thematically, and this really kept us in the experience.
Another thing they achieved, and this would have been through a LOT of testing and tweaking, is immersion in a state of flow. We all play games that we get lost in, but combining the brilliance of the game experience in HL2 with the flow that is achieved while playing it, makes it a really rewarding experience. I know I finished it, sat back, blinked a few times, and honestly felt like I had achieved something. If we get to the stage where we have put effort into something, and are rewarded for it, as with HL2, we enjoy it. During that experience we lose self-consciousness (I rarely thought about "Zac Duff" when playing...I WAS The Freeman), but we also gain, at the same time, a heightened sense of self. So, when we are playing, we wont think about ourselves, or check on a high level why we make decisions, but at the same time, we are being built up AS ourselves, and the low level decision making that occurs always affirms our experience.
I personally think HL2 is the best example to date of a digital flow inducing experience. Engaging, effortless, grand without being pretentious...the list goes on.
Greenlig