I recently downloaded the tech-demo level Lost Coast for Half-Life 2, and all I can say is ... wow! This level is a fantastic piece of work. The game play isn't anything new (you fight a few Combine soldiers, those ultra-freaky head-crabs, and a Combine gunship), but the technology behind the level is simply amazing. The high dynamic range (HDR) lighting is stunningly gorgeous; it makes the HDR technology in Serious Sam 2 look like something a child came up with. The effect is surprisingly effective, and it adds an incredible amount of realism to an already excellent game.
Half-Life 2 is one of only two games released in the past several years that really impressed me with its graphics capability (the other game being Far Cry). Half-Life 2 just feels real. And HDR only makes it more so. The wet sand on the beach in the Lost Coast level is truly dramatic, as is the lighting in the monastery. Also, the new character (the old fisherman) is so well done ... you just have to see it to believe it! Be warned that you need some serious horse-power to play the level in all its glory: an ATI x800 or NVIDIA 6800 (or better), 1GB of RAM, and a 2.9 P4 or AMD 3000+. My processor (at 2.8 GHz) is a little under-powered, but got the job done beautifully thanks to my 6800 GT.
The in-game commentary system is also an excellent addition. It was cool to hear the thoughts of the developers, why they chose to do some things and not others, as well as what technical challenges they faced. Hopefully they will include this feature in future games. All I can say is that I simply cannot wait for the Aftermath expansion; I've got to have more HDR!