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Rain is only visible at Shader level High or Very High, under Show Advanced Options in the Video Options screen.
This is a Left 4 Dead thing, not an IHM thing.
At Shader level High, you'll see some rain (although not exactly a thick storm, if that's the intent), and will also have flashlights casting shadows, some motion blur if you fling the camera around, your gun will leave smoke trails after firing, and I noticed the bump mapping on the ground and pretty much everything was higher quality.
My rig's not powerful enough for Very High, though, so I can't describe what you'll get at that level.