Huh, I also work in downtown Seattle but have never experienced any of this. I very rarely see shit/vomit, have never seen a needle on a bus, have never been spit at or attacked.
I've worked in downtown Seattle for the last 10 years, and frequently use the bus system. (caveat: I'm a white male, so generally in less danger, and may be more oblivious):
* I've seen a few brawls (~5) in all that time; generally (all?) swiftly broken up by police
* Haven't experienced weapons (or items used as them), whatsoever
* I've seen (a handful of) needles in that time, but never on busses
* Lots of panhandling, people yelling things, etc.
* 2nd/3rd around Pike/Pike is _definitely_ sketchy, and will make you feel uncomfortable walking around there; but it's also where I work atm: so I frequently am in that area, and haven't encountered much.
When working by the viaduct and Ferry Terminal our build slept a large number of people in the evenings on the sidewalk. One instance in particular stands out- someone had taken a shit into a 7/11 tall plastic cup, and I can only assume mixed it with some liquid adhesive, then flung the shit into a huge splatter against the sidewalk under the metal stairs down from the walking bridge. It stayed for quite a while, as the incentive to clean it up had passed- the building was being emptied to be demolished and security and other services had been stopped. But don't think this was a lone incident, as the back of the building would resemble the trope of the underside of a desk with boogers, except someone (or some group of people) were throwing shit (literally) against the back of the building.
You are fortunate and very lucky in this respect. But I would encourage you to stay situationally aware because the city is getting more dangerous and shit-filled.
I have been, twice in the last years. Spit on and punched at (the attacker was frail and tweaking, disturbing and menacing but physically unable to hit hard). Other good friends have gotten black eyes and so forth. This is in sf which has a more severe problem (though it is serious in Seattle).
This is to say nothing about day to day issues. Last time my wife ride Bart, a man got on and ranted angrily about “bitches”, though to nobody in general. In this case people just avoid eve contact.
It has gotten very very bad.