Things that can draw free agents....
No state income tax.
Challenge of bringing hockey (back after about a century, see Seattle Metropolitans, 1917 Stanley Cup winners) to Washington.
Expansion team also means no cliques are previously set, no roles previously filled. (That doesn't mean a guy will be guaranteed a NHL roster spot, if the organization operates on merit promotion; I expect a lot of guys to fight for one the first year. Organization may have a NHL penciled-in roster before camp and exhibition season that is very different from opening roster.)
An opportunity to create team culture.
Expansion also allows players to play with no "history", IOW a clean slate.
Amazon Prime Now and other Amazon services that often are tested in the Seattle area.
Warmer weather than many NE and Canadian teams experience (no snow).
Cons
Property taxes are above average. State sales tax 6.5% (higher than some areas).
Perceived "rainy" climate.
Demands on players for appearances, charity involvement. The newness may generate a lot of interest/requests for participation. (CBA does limit this somewhat. But teams usually have 2-3 events a year they "require" players to participate, such as team golf tournament, team evening activity, holiday hospital visits. But there is no limit on what a player might do regarding endorsements.) NHL teams often have a reading/literacy program with local schools. (FTR, AHL teams often focus on math and science. So if they are in the same region, there are different outreaches.)
Families having to figure "everything" out. There is no existing WAG group who know where the best schools are, best dry cleaners, restaurants, and all the other things folks depend on to "live". (While players often congregate in/around city/area where the practice facility is located, as opposed to the arena, school district lines may play a bigger role.) While there won't be WAG experience to draw on, local real estate agents (vetted by team), and some long-time resident employees, may help provide some of that initial local knowledge.