I'm a web-designer from norway and have been working with drupal for about 3 years now. Creating drupal themes is a big challenge, especially menus, tabs, blocks, nodelayouts and comments. Css takes a lot of time and can sometimes be a $"#$%&! Out of the box; Why create new themes? Why not do as the maintainers/creators of the quicktabs module? Within that module there are 11 different layouts, wow!. I would love to see a module that had several layouts/designs for comments or tabs. And if you had different menu styles and node layouts, one could pretty much create a more unique theme? Many drupal themes are so "typical drupal", blocks here, menu there. Personally I think this would be a timesaving and great feature for drupal. If there is a module that for example contains different tabs-layout give me a hint :D
Comments
Perhaps the question ought
Perhaps the question ought to be: why not create new themes? If you were to need a bunch of modules to get your layout right, I think you might have missed the point of the theme layer.
I think your idea of 'a big challenge' when creating a Drupal theme is subjective really. A big challenge compared to what? Creating a website from scratch? Drupal themes are not such a big challenge. Creating a <insert other CMS/CMF here> theme? Drupal themes are probably a bigger challenge, which is one of the issues the community is hoping to address.
I have a slightly different
I have a slightly different take on this, I don't think its that hard to create a Drupal theme, certainly no harder than any other CMS.
Whats hard (subjective of course) is modifying the output via the theme layer, which strickly speaking I don't view as really theming, but rather a powerful feature the Drupal makes available in the theme layer. Some of this is really easy, such as working with HTML in templates, some is really hard, such as wrapping form elements using hook_form_alter in a custom module.
Once you have the output sorted out applying the design to Drupal is pretty strait forward, so its a matter of collaboration between a developer/themer and the designer/themer.
For me the line between developer and themer is extremely blurred in Drupal, where the themer is expected to know how Drupal internals work and modify them using powerful theme layer tools such as preprocess fucntions, function overrides and custom modules.
So, I'm not really sure where this leaves us, but I tend to think this is just too bad - meaning that developers need to be highly aware of the theme layer, themers need to be more aware of the advanced features of the theme layer and designers would do well to learn the basics of the theme layer.
I'm all for Drupal being easier to theme, but frankly I'd be more inclined to want new features over easy, even if those new featues makes some things harder.