Almost 12 years ago when I was still a young teenager a group of ambitious people made history by publishing the first game of one of the most successful FPS franchises. On the race to publish a great modern FPS game Ritual Entertainment went forward with their SiN. As you know in gaming market the publishing date can have a significant impact on how well the game will sell. If you need to publish your game a bit later than similar highly successful game, it might not sell so well since everyone is already spending their money on your competitor's product. The impact of this fear was well seen in the race between Half-Life and SiN. In the end the difference between the success of these games turned into Half-Life's favor. Not because Half-Life was published on the better date but because it was finalized, polished product whereas SiN was released very buggy with lots of issues.
Other than being just polished Half-Life has been so successful thanks to Valve Software doing what it is so good in: a game that everyone can approach without the need of hardcore gaming dedication, unique storytelling spiced with humor and making the gamer feel like he (or she even) is in there in the middle of the action, living the story. You can just right hop in and discover a new intriguing world and before you even realize, your mind has been switched over from your boring day life into completely different realm, into a day when a laboratory experiment with anomalous materials went wrong having unforeseen consequences: the world started being taken over.
Regarding storyline, there was no real indication that the result of the Black Mesa incident would result in world taken over by aliens, this was only revealed in Half-Life 2 and actually the source of the takeover being this incident was only subtly hinted at Half-Life 2: Episode One when Dr. Kleiner makes his public announcement broadcasting. This shows the excellence of the story telling in Half-Life and its successors. Each game can be played on its own and they have distinctive storyline, yet when you play them all, the big picture starts to form piece by piece like in a jigsaw puzzle. The common factor joining them together is the main protagonist, Doctor in theoretical physics from MIT, Gordon Freeman.
You never hear Dr. Freeman speak. When you go talk to other scientists or security guards, they will respond to something... you must imagine the line Freeman would have used in that situation. This is not bad at all. It strengthens the involvement of your mind into the gaming instead of being just brainless shooting game. Here I must make a reference to books and movies. Many people have said that books are better than movies based on those books because with books your mind is doing the work whereas with movies all of the imagination is directly fed into you. With Freeman being silent it is left to the player to imagine what is going on in the head of this heroic scientist wearing a cool voice-synthesized protective suit with features such as "automatic weapon selection system" and "high impact protective armor" (another case of Valve humor). This has even brought a fan-made video series called Freeman's Mind, produced by Ross Scott of Accursed Farms production. I would recommend everyone watching it.
Of course at the time the game was made one could argue that the reason you don't hear Gordon speak is because of technology at that time and how much different alternative lines would have taken disk space and it would have been awkward hearing the same lines every time if space was conserved. But Freeman was left mute even in the later games and in Half-Life 2 the other characters even make some jokes of this, such as Alyx saying "Man of a few words, aren't you?" after meeting with Gordon. Maybe this was just to keep the sequel honest to this original game, or possibly Freeman was designed to be a mute person from the very beginning.
To conclude, Half-Life was a successful first person shooting game with game world anyone could dive into and has not received countless awards for nothing. Of course we should not forget how well the game base was coded making it one of the most moddable games of its time, bringing us such games as Counter-Strike and Day of Defeat that started as mere mods for Half-Life but which now sell for millions worldwide.