The biggest difference? Failure-rate.
Those 'enterprise' drives are warrantied for 5 years, whereas the cheaper ones are probably warrantied for less. Also look into the spec-sheets for them and look at their duty-cycles. The Enterprise drives are designed to run for 5 years straight, where the 'desktop' drives are designed to run 8 hours a day for 5 years. Very different use-cases and will impact your drive failure rates.
A second thing to look at is a line on those spec-sheets named "Nonrecoverable Read Error rate", which is a measure of the frequency of bits that are unable to be read inside the recovery window.
As of this posting (8/16/2011), the Seagate Savvio 10K.5, a 10K RPM Enterprise 2.5" drive, has its rate listed as 1x10^16. The Western Digital Scorpio Black, a 7.2K RPM consumer oriented 2.5" drive, has its rate listed as 1x10^14 bits. By this measure, the Savvio drive is two orders of magnitude more reliable.
This error rate puts an upper limit on how large of a RAID5 set you can build with such drives. When a drive fails in a RAID5 array, the array then has to read the entire RAID volume in order to rebuild the parity. If a non-recoverable read error occurs, you can lose the entire RAID set. Some RAID cards can get around this, others can't. They're not all built the same.
The above error-rate measures are approximate, but are the point where such errors are more likely to happen than not.
- 10^14 bits = 12.5 TB
- 10^16 bits = 1.25 PB
Only, you don't want to build arrays that large. The largest you want to build them is about 50% that size to minimize the likelihood of the rebuild failing. For those really cheap 1TB 2.5" drives, you can only fit 7 of those in a R5 array, where with the more expensive 10K RPM drives you could fit 15 of those 900GB drives in an array and feel safe in the knowledge that it'd rebuild just fine (but take a long time); your parity-losses are worse with the cheaper drives, which impacts your overall capacity.