I'm a relatively recent convert when it comes to Valve, the first game I bought from them being the Orange Box in October 2007. I bought it primarily for Portal, but the prospect of getting Team Fortress 2 (I was intrigued by it's Pixar-like art style) and Half-Life 2 along with the deal was a nice bonus (ironically, I play Team Fortress 2 religiously now while I haven't touched Portal for over a year). I had heard very positive feedback from some of my friends about Half-Life 2, so I gave it a try shortly after beating Portal.
Practically as soon as I started, I quickly became aware of a major problem: it's assumed that you had played the first game from start to finish. Consequently, the intro with Gman left me scratching my head in bemused puzzlement (I thought my video card was dying at one point with the weird screen effects). My confusion only got worse when all these strangers started talking about my character like he was a long-lost friend:
Barney: Hey Gordon! Remember me?
Me: Ummm...should I?
Alyx: Hey Gordon, it's nice to finally meet you.
Me: ...and how do you know about me again? Drop me a hint here.
Dr. Kleiner: At last! Gordon Freeman! Truly, this is a red-letter day!
Me: FOR THE LOVE OF GOD, CAN ANYONE TELL ME WHY THESE TOTAL STRANGERS KNOW WHO I AM?!
I managed to soldier on through, but I lost interest around the arduous hovercraft stages. I still had no idea what was going on, and gameplay was a bit too formulaic for my tastes. Despite this, however, it was clear that the game was very polished and revolutionary for its time. Because of this, I bought the original on Steam and started over from square one. Unfortunately, the game had not aged well with time, and I quickly got incredibly frustrated by now-archaic "find the key" puzzles. I came to the unpleasant realization that I had missed a watershed moment in gaming and that I would never be able to fully appreciate it.
I still have the utmost respect for the series, however, as it helped launch a plethora of other games and mods for the community. I would not be playing Team Fortress 2, Left 4 Dead, Zombie Panic, and Day of Defeat to name a few were it not for Half-Life.