The hype didn't start with HL2. It started with HL1, and you can always expect game hype to translate onto the sequel.
Half-Life came out in 1998, and at that time the genre of FPS consisted of simple shooting and walking. Half-Life hit the shelves like Katrina hit New Orleans and basically spun the genre into what it is today. In HL1 you had varied, specialized weapons, intelligent enemies (I don't think I've ever seen an improvement over the squad tactics of the soldiers and commandos), and an emphasis on puzzle-solving that was largely missing from most FPS titles, and that added a great dimension to the game. To me, however, what took the cake was Half-Life's focus on psychological pressure. Things were scary without being cheaply startling. The environment was claustrophobic, some of the monsters could kill you in two hits, the level design was so that it seemed dangerous and challenging to walk accross a simple catwalk.
Combine all that with impeccable pacing and an interesting story and you have what comes out to be arguably the best FPS ever made. (the graphics, of course, seem worse as time moves on, but you can't hold that against a game)
HL2, in my opinion, is not as great as the first, but does solid justice to its predecessor. Your ally(ies) are much improved in their character and AI, the graphics are still great even 5 years after the game was out, and the whole game played out a grand, arching storyline. To me, Episode 2 is what you should be playing to recapture the kind of quality HL1 had. It is my favourite installment of the new HL series.
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Aside from that, the game is made by Valve, and since Valve have come out with other fantastic titles like Team Fortress 2, Portal and Left 4 Dead, not to mention the creation and maintainance of Steam, it's easy to see why fans want to praise anything made by Valve.