There's two important things to keep in mind about Half-Life 2 (and often Valve games in general):
1. HL2 is dated in many regards and while it has been a pioneer, now many things have been copied and formed a new standard, but we can still look back at a classic that paved the way, just like classic literature. There are still a lot of things HL2 does "right" for many people that contemporary games fail to achieve, but you still have to keep in mind what it means to be a very influential series.
2. Story is told by the world and you have to actively explore, interpret and think, not read text or listen to dialogue. It's one of Valve's most characteristic game recipes and a major reason why many people enjoy playing their SP games.
If my describing of this storytelling technique is too vague, try diving into the Developer's Commentary. I remember Left 4 Dead had a commentary node giving a pretty direct example, somewhere in the first third of No Mercy if I'm not mistaken.
Also, people love the Gravity Gun and the characters, but that's even more subjective than the points I've made already, so I won't hold them up as central arguments. But the Gravity Gun is certainly able of mixing up the combat a bit, there's even an achievement for Half-Life 2: Episode 1 for only firing a single shot, so you'll have to rely on grenades and mainly the Gravity Gun for the entire game.
HL2 is not only capable of holding a giant fanbase over a long time without recent sequels or DLC, it also still generates new interpretations and debates about the game. If a game can keep a big community this busy and interested over such a long course of time, it most definitely did *something* right.
If you're in Prospekt Nova, you should've already been through a fair amount of story. You know your enemy, your allies, your goal and the world in general. The short-term plot is to rescue the leader of the Resistance, your sidekick's dad and your former colleague, the long-term plot is to fight alongside the Resistance to overthrow the oppressive alien government and - most likely - get to their leader in the Citadel, Dr. Breen.
That's a lot of plot actually (without mentioning what Gordon already went through) and most of it was conveyed directly via dialogue from members of the resistance.
I haven't played the first HL either, but HL2 shaped the world pretty much on its own and does a fairly decent job at filling you in if you explore a bit.
I bought the Orange Box at low price mainly for TF2, but I was also a bit interested in Portal, however HL2 and its episodes didn't catch my interest from the sound of it, despite hearing a lot of good in general on the net. I only tried them after I got bored with other games and thought "Eh, you bought em, might at least try em". It was the sort of attitude that usually ensures you won't enjoy the medium because of prejudice and you'll be looking for every mistake to verify your statement (and I'm fairly good at that I believe), but, well, here I am, to my own surprise.