Lets forget about overclocking and tethering for a moment.
I think the majority of users would be happy with an un-rooted phone as long as they could theme it.
I also think that the majority of users would be happy with an un-rooted phone if the OS was kept up to date.
-Mike
it sounds simple but its not. There are an infinite number of features users would have like to be included in the phone from the factory. But truth is that it costs a lot of money to pay developers to write codes for these features and implement them in a way that they will be almost 100% stable and will not interfere with other operations of the phone. Bugs need to be found and sorted out, and each additional feature offers new challenges and obstacles. In the end it all comes down to money and its just cheaper to produce a phone with the core essential features, and as much customization and flexibility as their budget allows for.
To speak to both points here, a great example of both addressing the theming topic as well as the issues that this brings up is Sweeter Home:
SweeterHome - Start - sweeterhome.com
This is a great piece of software that I use (and love). It provides a huge amount of flexibility in customization, and even allows users to download and try out themes.
But...
There is a learning curve. You need to be able to do basic editing even to downloaded themes. There are also issues in dealing with the behavior and form factors of different devices. Simply creating a theme for a specific Android build is not enough, you also need to factor in screen size and other issues.
This is a great piece of software that works brilliantly, but it has been in beta for ages, is still in beta, and does have some bugs (that you might find or not, depending on how you use the device).
A lot of this gets into what kind of "customization" you are actually talking about, what it takes to achieve that, and what compromises you make to get it. The basic level would be to offer the system as it is now-where you have a reasonably stable core OS and a user customizes it using apps and widgets and launchers. The next step would be to use a home screen replacement to get a specific theme, which can decrease stability but offer greater (albeit still somewhat limited) customization. Finally, you have the rooters, who want the ultimate customization and are willing to take the most extreme risks with their devices and systems (I fall into the last category, while my wife falls into the first).