Maybe you had to be there when it came out.
I must be missing something.
It took me a while before I could actually play Half-Life. I didn't have a PC with a 3D card until a couple years after it came out. I suppose maybe I'm just looking at it from the eyes of someone who's seen the fruits of the genre between then and now....but so many people seem to regard it as still one of the best FPSes around that I'm thinking that that doesn't matter so much.
Amazing it's not. Ground-breaking..maybe, but only just. What Half-Life is to me is a basic, mediocre shooter with graphics that hold up a little better than most games of that vintage. Much has been said about its incredible plot. I didn't see any such thing. There's a couple of levels where the setting is established, and then all hell breaks loose and you're shooting things or bashing them with the infamous crowbar for pretty much the rest of the game. ....or doing one of the innumerable, tedious, and badly implemented jumping puzzles, a game element that's horribly out of place.
There's the occasional scripted sequence, but most of them just set up some new threat you have to face without having any other real impact.
Let's face it. Doom 3 has more plot than Half-Life ever did. The reason Half-Life gets talked about for its plot is that unlike most games, it delivers what little there is entirely in real time. No cutscenes. This is unquestionably more immersive, but since you generally have little to no control over what's going on during those sequences, it falls well short of where I think Valve wanted to go.
There are many other shooters before and since that have more original settings, better plots, more exciting weapons, are scarier, have better designed levels, and generally deliver more gaming goodness.