The Mission is very different to visit vs. to live in. It reminds me slightly of Belltown in Seattle - nice to visit, kind of shitty to live in. If you're just there on Thursday nights a lot of the awfulness of the neighborhood fails to bother you.
Catching a giant whiff of shit and piss coming out of the BART station you can shrug off when it's once in a while, especially when you're on the way to a good night out. When it's every single day, to and from work, all hours of the day, it becomes tiresome. When you pass by a homeless dude passed out (maybe dead? who knows) on the sidewalk you can shrug it off, but when you see him slumped over every day on your way to work it starts digging at you. When you hear about some poor Mexican kid who got shot as part of a gang initiation, you can think "how terrible" and get on with your day... when you see the street-side memorials, and then you see another a week later, and then another, it becomes different.
The Mission is a different beast when you live there vs. just visit. If you decide to cocoon yourself strictly in the heavily gentrified western border of the Mission (because let's be honest, the Valencia corridor is a tiny sliver of the Mission), you will never have to deal with any of it. For many people though, the Mission includes everything east of Mission St also ;)
Not to mention, once you leave the bar and restaurant stretch along Valencia/Guerrero the whole thing becomes really shitty, really fast. Have you ever walked down Capp St at night? It's two blocks away from party central at 16th and Valencia. You really, really don't want to.
The Mission isn't Afghanistan, but it's objectively a troubled neighborhood that is among San Francisco's most violent (which is a small feat in itself, considering that SF as a whole has elevated violent crime rates compared to other major American cities). The Tenderloin tends to get a bad rap as SF's "worst" neighborhood - but its violence rate is actually not the highest in the city. Even more concerning is that the Mission's violent crime rate is largely driven by gang activity, which sets it apart from other neighborhoods of the city also.
You're still overstating the case. Yes, gang violence is prevalent, but its not like in Oakland where bystanders get shot for shit they had nothing to do with.
I think you should ask yourself what made you automatically assume I was the kind of person you're describing in your response to my original comment. Because this whole debate is constantly poisoned by idiotic assumptions on both sides.