Drives will wear faster the hotter they get. They will also wear faster if they are too cool. There is kind of a lukewarm operating temperature where you achieve the least amount of wear. I forget the exact number but I remember it being in the 45-c range (so just above body temperature).
Also got to remember that a 3600-rpm hard drive will wear less then a 7200-rpm hard drive under the same conditions. Not to mention a 3600-rpm hard-drive was the size of a small shoebox and took like 4-amps to run.
Quality and target market have something to do with it as well. Most desktop hard-drives are designed to be cheap at all costs. The MTBF for these drives is usually rated at like 20% duty cycle. This is typically what you are going to find at CompUSA and Staples, etc.
A mid-grade SATA drive like a Seagate ES or WD Raptor are designed for light-duty enterprise usage. I've been using the Seagate ES drives since they came out and they are very reliable and perform well. I have yet to have a failure. The MTBF for this class of drive is usually in the 60% area.
High-End drives, such as SAS and SCSI, are designed for the sole purpose of high usage and availability. The MTBF is usually rated at 80-85% duty cycle and they are usually given completely different internals as the desktop line. This is also reflected in the price. These drives will usually last YEARS upon YEARS in normal usage. Even in high-use raids they are usually only rotated every 5 years.
The other thing you got to remember is that MTBF does not mean the drive will last that long. That rating means that if you replace the drive on manufacturer suggested intervals (Life Expectancy) you will have that much time before you experience an unplanned failure.
Take for instance, a drive with an MTBF of 50,000 hours and a life expectancy of 2-years (consumer desktop drive). This means that if you replace that drive every 2-years, it will take 50,000 hours (Or 5.7 years) before you experience an unplanned drive error.
The consumer/desktop grade drives are only designed for a 1 to 2 year life expectancy.
The enterprise grade drives are usually rated at 5.
Again, this is also just a "guideline". Your mileage may vary.
