Anyway, I think playing Half-Life series through should be must to any modern gamer. It "raises the bar" (you know the book? Gordons crowbar? sorry for bad wordplay then)
I listed here few things what I find great in Half-Life 2 and its episodes:
* One of greatest stories in any game, it has very versatile way of telling story e.g. using visual desing effectively
It's at this point you realize the house has been all but hollowed out from the inside. A mortar shell juts from the basement floor, and what would once have been the ground level no longer exists, offering a perfect view through the building's punctured roof to the grey skies above City 17.
* --as the main character doesn't speak, one of my friend said that "well he is the lamest personality in gaming" but I just said it's something that makes it unique, you know all you need to know about him, he doens't talk because it isn't important.
Besides, who the hell would be good enough to be Gordons voice?
He's a scientist, but you know... he doesn't really want to talk about it* Innovative gameplay desing, HL2 mixes puzzles and well made fps game into great being.
* With the Half-Life 2, Valve introduced the best game engine of that time: Source game engine (it's still very good) which is widely used.
Good game engine was important aspect when creating large modding community and it brings longer life and versatility to Source. Many well made mods are taken to full game and retail, like Garry's Mod.* The atmosphere is great. You really felt like you were there, specially Episode 2 has given me new benchmark of impressive world desing.
The world is full of details and gives sense of reality. Sure, it's not the most realistic, but still it has given best feeling. I guess I just like the way Source simulates sun, athmosphere etc.
Also I especially love the mountain theme in episode 2
I would had awesome screenshots from all half lifes, but my pc is broken and couldn't get them...