I think a good example of the limitations of Mac can be seen with a recent customer I had. She brought in an iMac G5 which was pretty much done for, and decided that she wanted a completely new setup. After a discussion we determined that she definitely wanted a new Mac, but specs weren't much of an issue because her needs are basic, but she is older and her sight was going so we decided to try and get her the biggest screen available.
Well, the iMacs start at $1200 for a 21", which was bigger than her old one but she still thought she might want to go bigger for the screen. The only problem is the next size up in iMac starts at $1700. The other option was a Mac Mini, but the same size Apple display was $1000 by itself, which wasn't really any better. In the end we went with a Mac Mini and a 27" 3rd party monitor for less than the cheapest iMac.
The point is that because Apple didn't have a great solution for what she needed because they try and pigeonhole their customers to a narrow range of products.