Low impedance faults will cause a LOT of current, and will trip the breaker. High-impedance faults can still release a lot of energy, but if the impedance is high enough, it's won't trip the breaker, and the fault will continue unabated until something catestrophic happens. Proper calibration and careful setting of overcurrent devices is important in limiting the danger of high-impedance faults; EG, if you have a 1600A switchboard with 200A of load, set the OCP at 300A, not 1600A. Otherwise, you could have a 1400A fault, and never know it until the fire dept arrives.
For your second question, instead of solidly bonding the neutral to ground, it can be high-impedance bonded, where the neutral is still connected to ground and under normal conditions will maintain a potential very close to 0 volts, but under ground fault conditions, the high impedance reduces fault the maximum potential fault current.