Jason Snell says:

Mentioning Microsoft in this case is notable because, as anyone who has ever tried to buy and install a Windows upgrade package knows, it’s a complete disaster.

You have to have the previous version of the OS available — and heaven help you if it’s not the exact right SKU — in order to install the upgrade.

If anything isn’t quite right, it all blows up in your face.

And of course you need to make sure you’ve got not just a serial number, but the right kind of serial number for the exact SKUs you’re upgrading over and with.

In contrast: Apple. No serial numbers, full installer, no need to go back to previous versions of the OS in order to install an upgrade.