The first is the high degree of mental illness & civilian harassment per capita. In my experience, Seattle's homeless come across as particularly scary in their audaciousness and willfulness to commit minor crimes. Part of it may have to do with the car centric design of Seattle, which does not give you the NYC-esque safety in numbers. Whether it be 2 am in an empty-ish subway or a park with a lot of homeless people taking shelter, I have never been harassed in NYC. On the contrary, I seem to get confronted by a homeless person rather often when I walk through Seattle's most affected areas. FWIW, my intuition is that drug abuse is much more rampant among Seattle's homeless communities.
The most confusing thing about Seattle's homelessness, is that it coincides with the high foot-traffic touristy areas. In most other cities (Boston, NYC, DC); popular walkways, green spaces and core touristy areas tend to be policed and kept clear of homelessness. The problem persists, but doesn't directly interfere with your average civilian's life as much. In contrast, Seattle's most scenic and pedestrian-dense parts are more affected by this problem. Walking from one end of downtown (Pioneer Square) to the adjacent party neighborhood (Capitol Hill) is practically a walking tour of the city's homelessness crisis. I know a few people who do not leave their houses in downtown/pioneer square without a car.
I want to blame the last decade's incompetent city leadership. But then again, it is the people of Seattle who are voting for them.