OS X reinstall remembered my wi-fi password

As I understand your computer's system profile that contains that information is preserved through iCloud or by the servers when you do such a reinstallation. I've had the same experiences.

Note, that anyone who purchase the computer cannot restore the system you had installed unless they know your Apple ID and password. They will have to install an OS X they purchased using their Apple ID and password. That is why you have to completely wipe the drive before passing the computer on to another person.

Every wireless network password you use with every Mac or iOS device you use is stored in iCloud and linked to your Apple ID. You can delete individual entries from System Preferences > Network > Advanced > Wi-Fi > Preferred Networks.

I don't know why Apple considers it a desirable feature, but yes you can disable it if you want. De-selecting iCloud Keychain in System Preferences > iCloud will do it.

If you don't want to remember preferred networks, you can disable that feature on each individual device.

Also, what else besides my Wi-fi accounts is stored on their servers?

Other than the very essence of your being, that's pretty much it.

Well, John Galt beat me to the punch here. I guess he thought I wouldn't know the answer. 🙂

I apologize for any intrusion. Upon reading your response, it was not immediately obvious to me that this "feature" is linked to one's Apple ID, and not the hardware.

I don't think I even thought about it until this topic appeared. It was just one of those magical OS X moments that seem fine, and you don't think more about it.

Anyhoo, John, I'm not upset about your intrusion. I was giving you a bit of a tease, that's all. 😝