Yes, I realize that I am extremely late to the party... like, at least a decade late. But I wanted to give my thoughts on something that was still fresh on my mind (which was only a few weeks ago.) So this is going to feel like a broken record if you seen reviews of it already.
Besides that, Half-Life 2 had had a lot of thought put into making this an experience players old and new to the Half-Life universe and in first person shooters. As you are fighting your way through threats such as the Combine and zombies, it will become apparent that the star attraction is the Source engine, which was to be a physics engine that would be the most realistic for its time, and it truly delivers from how objects collide with one another to a weapon called the gravity gun, where you can pick up items and throw them at your enemies.
The game still keeps a level of consistency with how the game never leaves the protagonists perspective at all times, a thing that was new when the first game came out in 1998, there will also be familiar faces to those who also played the original Half-Life, so there isn't too much disconnection between the two.
The story is still given as much attention as it needs to be, after a while from the first game, a figure named G-Man awakes you because your time has come to take care of business once more, instead of Black Mesa, we have a different place called City 17, a place kind of like Nineteen Eighty-Four if it really was taken over in 1984 (it's still pretty unpleasant though.) Wandering around a bit will end you (Gordan Freeman) up in Dr. Kleiner's lab, and will be sent with Alyx Vance (daughter of Eli Vance) through a teleporter, at the last second during your transport however, the teleporter goes haywire and now are in great danger of being killed by Combine soldiers. Get out of City 17 as quickly as you can.
There's a lot more to it than that, but to reveal more will be a disservice to the game, it really is a game you should try out when you get the chance. If the story doesn't get you going, the gameplay will.