This is pretty accurate. There are better FPS games out there today (COD4 comes to mind) but the significance of Half-Life and its sequel are undeniable in shaping the whole genre. Much like Wolf3D popularized the genre, Half-Life refined it. Before Half-Life, the FPS was really just that: shooting tons and tons of shit and nothing else (Not that there's anything wrong with that idea, as Painkiller proves). If there was a semblance of a story, it would be told in gobs of text completely separate from the game itself. Half-Life broke away from all of this and was far more polished than other games of its time.Strafe Mcgee said:Mostly because both games have set new benchmarks for the levels of quality within the FPS genre when the were released.
That's the reason why it's such a phenomenon. Same reason why Ocarina of Time is widely considered to be so important: the things it did to Zelda and 3D adventuring as a whole set a whole new ruleset for such games.