Great article. As a long time user of everything, including System/OSX, I never thought about all the different ways because every OS was different. There are really just as many different ways on Windows too, though.
There's the Microsoft/Windows store (cough), direct from a developer's web site, from a file host site like Softpedia or C-net, or a file repository like sourceforge. You can have to run an installer or just un-zip a folder and run the EXE file with no registry entries.
Also, you may need to download the whole file or have to run just a download manager header that then downloads the file and installs it automatically (along with a bunch of adware/spyware). This always drove me nuts on Windows because it's then impossible to install it on an offline machine some place else. I don't think I've ever found this on OSX. Even stuff I've gotten direct from Apple was just a file (.dmg or .app) that can be moved to another machine.
I do think, though, that the whole DMG mount/open/drag & drop/un-mount thing is a little silly. I think the DMG thing came about because of Mac files becoming corrupted if downloaded on a PC, or transferred on FAT formatted media. Still, I'd prefer everything just came in a ZIP folder, and then drag the program to the Applications shortcut, or double-click a PKG installer.