1. Traffic sucks. Seattle and the surrounding areas have outgrown their infrastructure and the politiians spend a lot of time bickering over what to do about it.
2. LIVE NEAR WORK. You will spent the rest of your life commuting if you don't. If you have questions about specific areas in which to live, post them. There are people here who would be happy to answer them.
3. People are very friendly but they only want to actually MAKE friends in certain environments. If you join a hiking club, you will make friends. You will most likely not make friends at work, just acquaintances.
4. It doesn't actually rain that much, it's just overcast the entire winter. Whenever it rains hard, the Seattle people freak out. (NorCal here, and I think it's rather funny.)
5. Summers are great, excellent weather, lots of outdoors stuff to do, lots of events going on.
6. Arts and music community are really good up here. NWSource.com is a good place to look up events.
7. Definitely a left-wing, PC, PETA type county while the surrounding state is more right-wing, farmland, 2nd amendment type area
8. If you're conservatively religious you may have trouble finding a church/synagogue/mosque/etc. See point 7.
9. Good but competitive area for kids. Good schools are often full, camps max out quickly, lots of parents are pouring money and effort into college-prepping their youngsters.
10. Great outdoor activities like hiking, biking, camping, boating, kayaking, fishing, disc golf, rock climbing, skiing, snowboarding, etc.
11. Good work opportunities; MS isn't the only company up here and it keeps the market rather competitive.
12. Insane housing prices to start with, but the market hasn't dropped as far as it could have compared to other areas.
13. ? What else do you want to know about?