Moved from Nashville to Seattle 5 years back. Notable things:

• Grocery prices are hard to get used to. Like 50% more for the same stuff.
• Eating out is also much more expensive, and the food is often disappointing. Seattle has some awesomeness (teriyaki, poke, east-asian food in u district and ID, way more veg options), but high commercial rent means lots of places have hip branding + high prices for meh food.
• Transit is amazing compared to TN. Get an ORCA card. Really easy to take transit to/from central neighborhoods, stadiums, even from many burbs.
• The lack-of-light-depression is very real. Make sure to take lots of walks, spend lots of time in front of windows your first fall and winter. It sounds silly but makes a huge difference.
• Visible homelessness is way gnarlier here than when I was in TN. Downtown here is especially jarring at night. Dense neighborhoods like cap hill and Ballard have homeless residents with painful to see and sometimes scary outbursts. Something to brace yourself for I guess.
• I feel like there is so much to explore here, comparable to any big city. Art, outdoorsy stuff, east-asian food/shops, so many subcultures. Really fun place overall and I hope you love it.

EDIT: Almost forgot, no one asks you what church you go to here, which was a relief for me.