I have lived in Seattle for almost 13 years now (I also moved from Dallas), and as most have said the rain is really overrated. It tends to be drizzly from the end of September until June. But even then it's rare to get more than .5" of rain in a day. Winters tend to be mild (usually highs in the mid 40's with lows in the low 40's or high 30's). Summers are usually in the mid 70's with a few 80 degree days and any day of 90+ sending the city into a panic. Though this winter was very weird. Most days were in the 50's and the rain came in fewer but heavier events than normal. Some smart people say that as climate change takes hold, that will be the new normal.
It's also very casual. If you're wearing something nicer than jeans and a button down, you're already going to be in the top 1% and there isn't a restaurant in town you can't sit down in comfortably wearing chino's and a blazer/sport coat. Even at my company the guys high level management and sales guys rarely wore more than OCBD's and jeans.
Here's my basic wardrobe. For the winter (starting in October) I usually wear a button down shirt with dark denim, chinos, five pocket cords, or five pocket moleskin. I rotate three pairs of shoes. I have an Allen Edmonds Long Branch in golden chromexcel, a black chisel toe pair of Blundstones, and a cordovan Chippewa Moc toe work boot. I also keep a pair of Blundstone work boots around if it's really raining or there's snow on the ground. I tend to wear either a button down shirt or a wool sweater during the winter. On top I have a black Patagonia rain shell. I keep meaning to buy a duffle coat to replace my very old winter coat, but I haven't needed a winter coat the last three winters.
When I'm dressing up for dinners in the winter; I wear moleskin, flannel, and tweed trousers. Any shoe I will wear for the winter either gets a topy (my Alfred Sargent Longwings), comes with a v-tread (Brooks Brothers AE Park Avenue), is danite (Meermin chocolate suede wingtip oxford), or synthetic sole (RM Williams Blaxland). I usually wear a tie and a cardigan, v-neck sweater, sweater vest, or a sport coat to dinners; but I'm often the only one in the restaurant so dressed.
While our summers aren't hot, something to keep in mind is that many places won't have A/C. I've never lived in a house or apartment here that had it. Most restaurants (unless they are in new construction) don't. Even some of the schools don't. So I tend to dress like it's warmer than it is. Lots of linen shirts and lightweight (poplin, oxford cloth, or seersucker pants). I also don't worry too much about topys on any shoe that I will use mostly for the summer.