There are people who just want to blog and there are those who are willing to spend time learning css, php etc to make and mod themes etc. I probably speak for a lot of bloggers of the first type that want to be able to change headers, sidebars, colors etc but don't want to spend time learning css or any other language needed to make these changes.
If theme developers who provide the ability to mod their themes spent a little time to create an app that bloggers can use to change colors, header images, sidebar widgets etc etc it would grow the WP community even faster than it is growing now. An example I'll point to is Cutline by Chris Pearson. In the version up at WP.com I was able to change the header image easily. In the hosted version, his theme is way more powerful in that I can change virtually every part of the theme but as I said before I don't want to spend the time, if I can avoid it, to learn all these new languages.
I know its a labor of love for a lot of developers who put up their themes for free but I am sure some of us would be willing to pay for themes that can be modified w/o having to go thru the the learning process of a new language or spend time incessantly in help forums bugging experts.
This is just an idea so don't shoot the messenger :)