SLU is a former industrial neighborhood now filled with Amazon offices and cookie-cutter apartments. Basically the positive there is that you're close to work, the neighborhood itself is severely lacking in culture.
Maybe I've just been desensitized, but I really feel like there is no cause for alarm in Belltown.
In the past year I've seen ambulances taking away normal citizens who've been beat down (twice) and I've had a stabbing perp run by me as he was chased by the cops. And this is with me almost never going to Belltown. I've never seen anything remotely like that while I lived in First or Capitol Hill (although I've definitely seen plenty of car break-ins).
I respect that other people have different tolerances for urban annoyances.
It really hasn't. I'd be more worried about living in the U-District than Belltown or even the International District.