Dear Esther used a different, optimized and better detail sprite system. The system could hold up to 3 times more stuff than the original. Also, I think the detail sprites were somehow linked with the fog and some particles, so that they all move uniform with the wind. For questions ask Mr. Morgan, alltho I doubt he will respond in a timely fashion nowadays.
I have no idea how source does it, but you can usually write a pseudo wind shader as a vertex shader. I think it's a pretty standard thing. You can go have a look at overgrowrh, by wolfire games. All their shaders are editable if you download the game.