For all those scoffing at the idea of Mac Apps being difficult to install now, let's just review the possible options:
1. Download the DMG, double-click to mount (if not auto-mounted). Drag app to Applications folder (sometimes via a provided alias, but usually not), then unmount Disk Image and delete DMG file. Ease of use: Good. Intuitiveness: Not so good (especially for users coming from Windows). I can't even count the number of times I've seen someone running Skype or Firefox from the disk image every time, and then they trash the dmg file at some point thinking it's just a useless extra, and suddenly their app doesn't work any more. It's not a bad method, but it doesn't make sense unless there's good instructions in the download (some developers do a good job here, many don't).
2. Download the DMG, double-click to mount (if not auto-mounted). Double-click installer, follow directions. Ease of use: Good. Intuitiveness: Good. I think this is the best option currently, though of course the argument is that an installer is overkill for most apps.
3. Download a zip file, double-click to decompress, then drag resulting app to the Applications folder. Ease of use: Good. Intuitiveness: Fair. While this doesn't have the issues of the similar DMG method, it still doesn't automatically result in the app being installed correctly into the users Applications folder. Doesn't affect all apps, but it's still good practice for them to be there.
4. Purchase on disk, bring home, place in disk drive, run installer. Ease of use: Fair (only because of inconvenience of going to the store to purchase actual disk). Intuitiveness: Varies, but usually good. Adobe's apps get a poor rating in this regard, however, as their installer is a layer or two deep into the disk.
So while none of these are exceptionally difficult, the array of options (and I'm sure I missed a few) makes it a confusing process for users who are not tech-savvy, which is by far the majority of the population, and also the target market for this sort of product. The Mac App Store simplifies all of these steps into a simple click to install.
jW