Apple didn't let people do much with their iPhones... so the hackers came along and let people do too much. As an example, here is an iPhone theme that two of my friends use and think looks cool:
The problem is that some people abuse their ability to customize how something looks. Take MySpace. Every element of a MySpace page can be customized, and most people don't know how to use this to their advantage. Good example below:
Sites like Facebook, however, keep every profile looking decent by using a consistent look. Facebook's problem, however, is the hundreds of Facebook "apps" that people add to their profile.
Lesson learned: Too little customization is bad, but too much is worse.