HL1 (and 2 I guess in some places) had this eerie, quiet "lonely" silent feel about them which I always liked. It's hard to explain, and perhaps I'm talking out of my ass, but in HL1 although occasionally you found a scientist/guard, mostly you were completely alone in these sometimes large, open environments that gave the player a certain feeling.
Not very hard to explain, really. As much as the scripted sequences with NPCs are lauded, I'm pretty sure the majority
of levels are spent entirely with you against the Combine. Those are the moments that make me feel like a badass. There are scripted bits along the way, sure, but assaulting near impregnable bases with giant space bugs? That felt like the only
time in a game where I felt using cover was actually effective. The design is still sweet.