I agree with the whole list.
On #4, the game really does feel open! They subtly guide you to follow the proper path, and only when you deviate from that path do you feel it is closed. They did the same thing with Half Life 1.
One more thing: The variety of gameplay. On an airboat, in a dune buggy, fighting a flying ship on a bridge, and one of my favorites, crushing baddies with a huge crane (I know it was only for a few moments, but it was fun because the physics engine is so awesome), fighting zombies, etc etc etc. This is one thing that's a little bit missing from the new episodes (well, I've only played Ep1, I still have to play Ep2).
Half Life 2 was an incredible game and I only hope the hold-up on HL3 is to make it just as incredible as 2 and 1.