I've come to the unquestionable conclusion that Apple doesn't want customers inside their computers. Full stop. Apple views the personal computer as a device to be used for its serviceable life (until a major component fails), then discarded, recycled, and replaced with a new one.
I'm not defending this stance, btw, just stating what I've observed. Our household recently experienced a HDD failure in an iMac purchased late 2010. At 4 years old, the HDD was due for a failure (based on my experience with spinning magnetic disks). No problem, I thought. I'm an experienced hacker; taking this thing apart will be no problem at all.
I was right in that taking the iMac apart would be little challenge for me. Honestly, the iMac is a miracle of serviceability for such a compact computer. All of the connectors unsnapped as expected while using the correct tools. Nothing was terribly inaccessible, with the exception of the LCD driver cable. That connector terrifies me, but everything else was cake.
What killed me was the HDD thermal monitoring system. Apple uses the internal HDD thermal sensing equipment. Ostensibly (and probably true), monitoring via SMART requires an interrupt, which momentarily halts HDD communication. I/O on a HDD is bad enough without these interruptions, so hats off to Apple for prioritizing performance.
The down side of this is that there is no standard for the thermal sensor pins in the connector provided on the HDD. You have to 1) make sure to buy a HDD of the same make as the one you're replacing, and 2) cross your fingers that it isn't a WD HDD, which apparently went through a running change, which A) flipped the thermal sensing pins, and B) changed the connector subtly.
For some context, have a look at this article:
http://blog.macsales.com/19617-diagnosing-2009-2010-imac-fan...
I experienced two challenges. First, my connector had a "nub" on one side, which fit in to a slot in the port on the OEM HDD. The new WD drive had no such slot. Second, my HDD temperature sensor cable had an 8-pin connector. In the article above, you can see that the connector is a 6-pin connector in an 8-pin hole. Because my connector was 8-pins, I couldn't simply flip it, because that shifted the cables by 1 column. I ended up hand filing the nub off the connector, then literally grinding and filing one column of the pins off the connector, turning it in to a 6-pin connector, so that I could flip it.
The moral of the story is that Apple makes zero considerations for end-user serviceability. None whatsoever. I wouldn't go so far as to say that they intentionally make it difficult, just that when faced with any decision where the choices are A) benefit the design or profitability of the device in some way or B) retain end-user serviceability, factor B never even gets considered.