It really can. In fact guides and articles have been written on the subject, many of them over thinking things. But really, take a step back and inhale deeply, and it will all seem a bit simpler.
First, keep in mind that service employees in Washington state are paid minimum wage (at the very least). Why is this important? In most other states service employees get paid a certain amount less than minimum wage, and thus need said tip to earn minimum wage. This is not the case in Washington state.
That shouldn't be a reason for you to be a heartless bastard, of course. Minimum wage in itself is not cool. But it also means a tip doesn't need to come automatically.
So what should you tip?
At a sit-down restaurant: For normal good service, tip between 18-20%. And if math is not your strong point, simply take 10% of the total (you know, by moving the decimal one spot to the left) and double it. Now, it is legal to go both up and down, and frankly, if the service stinks, then don't tip. Keep in mind, though, that "stinks" means "really really really really bad service." I can't think of the last time that happened to me. And if the food and service is stellar, hey, an extra 5% isn't going to kill you.
Barista: Tip if you're a regular and receive good drinks. A buck should suffice, two depending on how complicated your drink is. Although if you're getting five triple macchiato semi-chilled latte frappuccinos then you should obviously tip more. Round up to the nearest dollar for coffee shops you don't frequent if you feel the service was particularly good.
And again, if you don't receive good service -- not uncommon in many coffee shops around here -- don't tip.
Bartenders: Adding a buck is pretty common for beer and wine or straight pours, two for mixed drinks. Adding a bit more is fine if you're a regular or ordered a complicated drink.
Delivery: Anywhere between $3-$5 is standard, depending on how much food you have delivered.
Fast food: Would you tip at McDonald's? No? Think about it, do fast food employees work less than a barista? I mean, it's up to you; conventionally you wouldn't tip a fast food employee, but would it hurt that badly to throw a buck his or her way?
And when in doubt, always follow the master: