Also, the vehicles are vastly improved. In Half-Life 1, it was just a 'train' on an invisible track, that could only move forwards and backwards at three speeds. HL2 has actual vehicles now, so you are free to roam on them wherever you wish. I've actually had a go on a car in the game, and they roll over and all sorts, and it's not a simple case of just turning, you do need to watch your speed or you can fall off a cliff edge or whatever.

I've also seen an in-game video of the same car, and the player almost fell off the edge, but managed to climb out and hit/shove the car back to safety before returning to the road.

The skies impressed me, too. The clouds move and stuff, it looks perfect.