Like real estate, it is about location, location, location. Think of it as a lever - if properly applied, it can move a world.
There's also the emotional component. The Japanese population has been shaken by the initial quake, the tsunami, aftershocks, and the problems at the nuclear reactor. Adding another significant quake and tsunami (2m, last report), will further rattle already stretched nerves.
Anything over about a 6.5 or so is a pretty big quake. The '89 bay area (Loma Prieta) quake was just a 7.1 (and struck 10s of miles from downtown San Francisco). The recent Haiti earthquake was just a 7.0.
It depends on many factors. I was in a 6.8 earthquake in Seattle in 2001 that did some fairly significant damage to a few buildings in Pioneer Square, but otherwise was just a scare and an inconvenience. At the time I read that we got off "easy" because of the depth of the epicenter. An earthquake of similar magnitude had hit Turkey a few years before that and did a lot more damage.