Basically, as said before, it comes down to heat. Like measuring fan loudness, manufacturers rate their drives based upon having it on a workbench with open air around the drive.
There are two ways to handle this.
1:Get low heat drives that spin as slowly as possible. Laptop drives are actually ideal for this and put into a typical desktop machine will never generate enough heat. They will almost always die when their electronics or bearings go out.
2:Get a fan. I have 300 gig Maxtors in my old system. These were the hottest drive ever, or nearly so. Horrible reliability, yet mine are three years old? I have my intake on my computer sucking in air over the drives. I removed the drive bay cover and turned off the intake fan. They get up to maybe 85 F(!). Put these same drives in a typical tightly closed box or server room and dead in 3-6 months, tops.
Any drive kept to under 100-120 F will last essentially forever. It's often cheaper to buy a good fan than special low heat/low power drives.
That said, there are only two companies of note in the drive business any more. Hitachi and Western Digital. Western Digital is better overall, IMO, because they use more advanced technology, which means in some cases, single platter drives. The GP series isn't single-platter, but they spin at 5200rpm and are very quiet and cool. Blow a little air over them and you'l never have a problem. Hitachi is a bit better in the laptop area, IMO, though WD doesn't make bad drives, either.
That said, important stuff should be run in raid 1(redundancy mode). Drives are cheap enough now that you really are smart to do this. The chances of both drives dying at once and both being unrecoverable are virtually zero. Cheap insurance for as little as $60-$80 extra(cost of the second drive).
I just set up a new machine this weekend. Two 160Gb drives. Raid 1. Fast and stable. $65 each from WD.
Note - if you want to run Raid, use special drives meant for it. These often cost as little as $5-$10 more than the consumer models, yet have a longer warranty and extra features. WD's Raid drives aren't meant to run as single drives, though, and vice-versa. Their Raid drives are the Raptors and the RE2 currently. Their consumer drives are their GP series. I used RE2s for the Raid(system) and a GP for the data drive. The program and data I can recover or reinstall(it also gets backed up). The system, if it goes, I'm down for two weeks. This way I suffer no real downtime.