I moved to Indianapolis. I don't know if it's bigger than Seatle, but it's certainly bigger than anything in Iowa.
Traffic here is not an issue. It can be slow moving occasionally, and there's construction a lot, but it's not Chicago or L.A. I doubt Seatle will be horrible, unless you get aggrivated by not being able to go faster than the speed limit at any time of the day. I've never had my car broken into, either, and I've been in the shadiest parts of Indy and drive a VW (not a great car, but it's still nice and in excellent condition with a CD player in it). Keep in mind that there's cities like Chicago, New York and L.A. and then there's cities like Seatle and Indianapolis. They are NOT the same as the BIG cities.
The only things I really miss are my family, ISU games and the Iowan people. You really won't meet any better people, and you'll realize that when you move.
I have a couple of friends that live in Seatle, and they belong to the ISU Alumni group there. They do stuff once a month or so, so that may be something to check out so you can even get a little taste of Iowa all the way on the west coast.
Good luck in your travels!