I think most of the complexity is around the concept of disk images. If the app was just distributed in a zip file (like many in fact are) then you just unzip it, and the application is right there. You can move it to /Applications if you want (and many apps offer this) but it's not necessary.

Disk images seem to be much rarer on other platforms, and are usually only used for images of optical disks. Even the concept of "ejection" can be confusing, especially since when mounted disk images appear almost identical to actual physical removable media. I'd like to hear the reasoning behind why mounting disk images isn't relegated to a vestigal "power user" feature by now, since it seems like zip files (especially in appledouble format) can serve the same needs for most normal user flows.