I moved from LA to the Seattle area about 4 years ago and ready to move back now (not to LA specifically, but California). Here's my quick take on some of the differences/observations from a native Californian:
Great air (no smog!) and water quality. Very green, lots of beautiful nature all around.
Lots of outdoor stuff to do, hiking, biking, ski/snowboarding nearby, many bodies of water for boating/watersports.
Great motorsport tracks nearby as listed in an earlier post. The Ridge is a brand new track and it's amazing, like a NW Laguna Seca.
Plenty of opportunities to drive tracks in the wet and learn how to do so well.
People are friendly, cities are very clean/well maintained.
As stated earlier, the summer weather is great from July 4th (literally, on that day) through October.
On the flip side, November through July (9 months out of the year) it's incredibly dreary. It's not the rain, it doesn't really rain hard that much, more of a drizzle if anything. It's the constant overcast/dark cloudy weather that's gotten to me. After a while it definitely can affect your mood. I have a sun lamp on my desk at work. This is the main reason we're moving back.
Miss the ethnic diversity and food variety. Don't get me wrong, you can find most things here, just have to go into Seattle itself usually but not the same as what you find in LA. I miss the street fruit stands over there, it's the little things. :)
I miss the southern California beaches. There are lots of lakes here but it's not the same (I live more inland, can't say about the coast here).
So overall, nice place to stay a bit. I came here because of work but we're glad we didn't buy a house and are still renting. Makes the move back much easier.