what affects lifespan
the RPM
how fragmented files are
how often it is accessed and/or changed
the PSU may fluctuate and cause the hard drive to burn out, or simply "screw itself"
it may overheat due to lack of cooling, more so on SCSI drives
the angle it is on; if it is on a funny angle, the platters may 'weigh down' on one side, this can cause problems
transport/rough handling
the quality of the drive itself:
if it isn't sealed properly it can get dust in, one piece of dust can make a hard drive's read/write heads to crash into the platter/s
if the platters are uneven they might cause vibration and/or hit the read/write heads; a lot of the time the vibration itself will be the cause of it crashing