Adam We to the rescue again with some epic half-life2 knowledge! (might of been said before but honestly TL;DR)
Half-life's popularity comes from the fact that it had all the aspects of a good old style FPS while keeping a fantastic storyline, puzzles to keep the user thinking and horror and survival theming without succumbing too - graphics rape,a dicked storyline,or random in your face spookarama or what I have come to call FEAR 2 syndrome.
half-life 2, Like half-life was a benchmark in gaming. A new level of gaming standards set by the valve team, even today valve still blaze a trail brighter then a nobel prize winner on a red bull high with Portal.
When half-life 2 was released it was considered "epic graphics" and even today they are still impressive (to me anyway). It was one of the first games to harbor HAVOK physics and using this new power valve had built the game using an excellent sequence of storyline, mystery, fights, puzzles, horror and the usually quite immersive survival element.
In short the reason for half-lifes popularity is because it is not only good but it appeals to everybody, Ravenholm had me pissing myself the first time, and by Dark Energy I was so immersed I found myself caring for the characters (which is rare!). Half-life 2 gives you a shot at them all and pulls it off with a finesse that even todays modern games still cant match. (seriously, like wtf is with the industry, its like valve is the only mainstream company thats taking the consumer into consideration. The rest are just doing it for cash)