It's hard because it's far more complicated than clicking an installer. I outlined the procedure. Anyone can do it once they understand it, but how would you ever guess to mount a virtual drive (most people wouldn't even understand what one is) and then the rest?
.dmgs threw me for a loop the first time I tried a Mac, as well. If somebody had just told me "they're compressed loopback devices" I would have understood (I was coming from Linux), but I was expecting them to be like tarballs and spent a while confused.
I think it is only hard because your brain is thinking in terms of virtual drives and installers. For my grandma and my 11 year old cousins both types of installs are simply clicking various things on the screen until stuff works and are essentially equivalent.
My brain is thinking in terms of click the file and click ok vs. click something, figure out how to get the application, figure out where to drag it, do so, then try to get rid of the original file.
Some installers make the process easier with the little window that shows you both icons and tells you to drag the program icon to the applications icon. That's still far more complicated than Windows, but a lot better than most Mac programs, which don't even give you that.
DMGs auto mount by default if you use Safari (which the kind of people you're worried about would be doing). When they do, the window also automatically opens presenting them right away with the application.
In the best case, they could just click on it and the right thing would happen. This works, but the results aren't what you'd expect usually, since the app will go away once the dmg is gone. The exception to this is Delicious Library 2 (unreleased), the only app I know of that will tell the user the app was loaded from the disk image, and offer to put it in /Applications for you.
In the second best case, you are given the shortcut, and you manually drag it yourself. Then you click applications, and then you click the app. Perhaps a little convoluted.
In the worst case(and by worst I mean, worst assuming the standard default configuration), you aren't presented with the applications folder link and you end up dragging it anywhere -- perhaps the Desktop. But, if you do that, the app still works! That's the beauty of the app bundle, it doesn't matter where you launch it from.
Could the whole process be refined? Yes. And developers are actively making it better (just like they are actively making things like upgrades better with Sparkle). I for one, though, still think its much more friendly than any windows installation with its ridiculous wizards, shortcuts added to four different locations, and the mess that is the windows registry.
Apps packaged in DMG files is sort of an odd artifact of OS X's past. Early versions of OS X didn't really have support for various archive formats in the GUI, and so the DMG format was the best option that worked out of the box. Now with 10.5 you can use a whole plethora of archive formats. Heck, you can send someone an app in a .tbz2 and it will unpack as expected with a simple double-click.
(I may be remembering this all wrong; someone please correct me if that's the case.)
