Why does pressing the red X close some windows, and seemingly minimize others?
Mac OS has always had a multiple document interface. Applications like TextEdit can have multiple main windows open without requiring multiple instances. The open command doesn't usually open documents over the current document.
The close button might also quit the application after closing the last window like on Windows. But what if you want to create a new document? Many applications create an untitled document on startup. Should closing it quit the application?
There's no real harm in keeping applications running in the background. They don't usually use CPU, and you should have enough free or inactive memory anyway. Quitting and opening applications all the time would take more time. OS X has also supported automatic termination since 10.7.
Automatic termination eliminates the need for users to quit an app. Instead, the system manages app termination transparently behind the scenes, terminating apps that are not in use to reclaim needed resources such as memory.
Automatic termination transfers the job of managing processes from the user to the system, which is better equipped to handle the job. Users do not need to manage processes manually anyway. All they really need is to run apps and have those apps available when they need them. Automatic termination makes that possible while ensuring that system performance is not adversely affected.
Apps must opt in to both automatic termination and sudden termination and implement appropriate support for them.
The close button does actually quit some applications with a single main window, like Dictionary, System Preferences, and Calculator. Opening them again doesn't take that long and quitting them frees users from having to do it manually.