I try to visit Yosemite every two years because it is truly one of those special places in the world.
The drive from San Francisco is about five hours and the roads can be white-knuckling but beautiful.
My first impact is wow plate tectonics are powerful. You can understand the power of the earth when you visit Yosemite. It doesn't have the robust wildlife like Yellowstone, but it has majesty in abundance.
The valley is a great visit because you are surrounded by some of the highest rock cliffs in the world and you can, in the spring see and hear some amazing waterfalls where you can feel the power of the water slamming onto the rocks far below. (photos of the valley: http://www.flickr.com/photos/mcgrayjr/sets/72157606886366612/)
And to watch people climb El Capitan and see their lights on at night as they sleep hanging onto the cliff, truly amazing.
I heard wolves howling at night, which I always find to be a beautiful sound.
The days during the summer can be pretty hot, in the 90's, but you can enjoy the slow moving streams and rivers because the waterfalls are just at a trickle.
If you've done the valley, next try the high Sierras as that is a completely different feel and experience where the lowest points are still over 6,000 feet above sea level! The hiking is spectacular and you have times when you never see another person other than your hiking partner (never hike alone!).
Yosemite, an American treasure!