Couple of things I'll mention, some of which have been covered, some have not.
1. Luck of the draw - Every year a certain amount of peeps are struck by lightning. The % is so small that ya would think, no one's ever been struck twice .... but it happens.
2.a Thermal Cycling (electronic) - Of all factors, this has probably the biggest effect on the electronic parts of a HD.....more so even than heat itself (within reasonable limits). The expansion and contraction due to thermal cycling puts stress on silicon and soldered connections. After enough cycles, these connections sometimes just go. Kinda like a light bulb where the life depends not on how many hours its lit, but how many times it gets turned on and off. This can be minimized by letting the PC sleep instead of turning it off.
2.b Thermal Cycling (Mechanical) - There's also a mechanical component of the above. The different parts of the HD heat up and different rates and again die to thermal expansion and contraction, this affects the tolerances and clearances between rotating and moving components. Again, this can be minimized by letting the PC sleep instead of turning it off.
3.a Heat (HD iteself)- This covers a few areas. When ya studied the stats on the HD's you were considering, did ya look at their running temps ? Here's a comparison of 1 TB drives.
=on&prod[3016]=on&prod[2365]=on]http://www.tomshardware.com/charts/2009-3.5-desktop-hard-drive-charts/compare,1015.html?prod[2371]=on&prod[3016]=on&prod[2365]=on
Seagate 7200.12 - 38.00
Spinpoinbt F3 - 41.00
WD Black - 43.00
Lower temps obviously is an advantage. A HD in a well ventilated case will generally run no more than 10C above internal case temps.
3.B Heat (Case Cooling) - Typical; case cooling draws air in over the HD's and "then" it gets to the CPU and GFX cards producing the major heat. In an appropriately sized and cooled case anyway. When ya get chinsy on the case, everything runs hotter....a 10C temp increase can reduce life by 50%.
4. Vibrations - Bump ya case much ? got the bass turned up to the max ? Use vibration isolators on the HD mountings ? All will contribute to HD life.
5. Power Stability - While everyone always asks if their PSU is big enough, no one ever seems to ask how stable the voltage it supplies is. ATX standard is 5% max ..... I look for 1%. Also, and especially if OC'ing, how stable a voltage does the MoBo's VRM provide ... look for multi phase designs for the best stability.
6. Brand Name Reliance - I'm always amazed how many peeps judge an entire brand by one component. If GM sells a $17k car and an $89k car, would anyone expect them to be of similar quality and reliability ? Yet with PC components, that seems to be the case more often than not. One way to judge the reliability of a component is by looking how one party who knows more about it than anyone else judges it. Ya can do that by the warranty. When a manufacturer offers a chinsy warranty, that tells me that the manufacturer doesn't have a lotta confidence in his product. Of course, a manufacturer can bump up the price to cover a large failure rate's warranty cost but failed costly HD's tend to cause more ire than cheap ones. But if ya got the warranty at least it doesn't cost ya anything.
I have seen my HD failures drop considerably since paying attention to the above. I have a NAS w/ (4) 7200.12's running 24/7 and it's been running 5 years w/o a hitch. I have a file server running 15k Cheetahs 24/7 since 1998 and they on their 2nd set of HD's. Again, here the fact that they never thermal cycle, and are very well cooled wit stable voltage supplies has helped.
On the desktops, I'm only using full tower cases with 8 - 10 fans, all on their lowest possible setting. All cases have dust filters and anti vibration feet on case, fans and HD attachments. All PSU's are 10.0 jonnyguru rated and all MoBos are 16+ phase designs.
Another note on brand names is that every manufacturer has had some duds..... Seagate's 7200.11 was plagued with bad firmware but it's 15k Cheetah holds the No.1 ranking with drives going 10 years. The infamous Raptor's WD1500 showed a 25% failure rate within the 1st 2 years on storagereview.com .... the newer ones did much better.