There really is a difference. However... that difference is in the firmware, and that's where the manufacturers were scamming.
There is a thing called TLER - time to recover from errors. That is, if the drive is trying to write or read from a block, and it finds a problem, it goes to write to another block, or uses error recovery to read. For servers, they really want the time that the drive keeps trying to be under seven seconds. The consumer drives could be adjusted using software like hdparm.
Then around '09, and apparently starting with WD, they made a change to the firmware, and you could no longer change that variable. Servers scream and gag and give up, and tell you the drive's dying, when instead of spending under seven seconds, the drive keeps trying for ->over two minutes-. Everything you read says do *NOT* use those for RAID, either.
The server grade drives are very much *not* into spinning down, and they have that short TLER.
This is why, around here, we're ecstatic at the new WD Red drives, that are "targeted towards NAS"; the reality is that they've got TLER set to seven seconds. And, where the server grade drives are two to three times the price of consumer grade drives, or higher (some sources are a *lot* higher), the Reds are 1.33% of consumer grade drives.
Reliability: we have some of everything, and have not noticed a real difference in reliability. And our drives get used a *lot*.