I enjoyed reading everyone's ideas. There are a lot of good suggestions here. Here are my thoughts:
The default WP theme first and foremost should serve as a great starting point for new themes as Kubrik is today. The css tags and layout/structure should remain similar in the new version. I don't think the default theme should be a complex theme framework. I do think that it should demonstrate what is possible to do with WP, such as displaying horizontal lists of both categories and pages, thumbnail images, and several sidebars.
The default theme should define the sidebars that most themes will support. Just because there are lots of sidebars does NOT mean they all need to be used. Rather they serve as a way for WP to be used more like a CMS for those that want to. If nothing is added to a sidebar, it wouldn't display. Eg if Left Nav and Right Nav 1 was left blank, you'd have a traditional single right-nav layout.
* Header (add social networking icons, ad block, etc.)
* Page Top
* Main Content Top
* Left Nav 1 (doesn't show if empty)
* Left Nav 2
* Right Nav 1
* Right Nav 2
* Main Content Bottom
* Page Bottom
* Footer (add nav, popular posts, recent comments, etc.)
I'd like the theme to be fast and if adding options for fonts, colors, etc. will slow it down too much, then I'd suggest just putting DEFINE's all in a single location in the theme so that people can easily change them there. Note that options for number of columns that people have suggested wouldn't be needed if you use the multiple "sidebars" approach above.
I'd like to see the default theme include a layout for every layout that exists (home, author, etc.). People have suggested including multiple themes, but I think we just need multiple home and page templates. For example:
* Home - Magazine Style (thumbnails, large format for sticky or featured posts, smaller format or just small thumbnail and title/date/comment count for articles by category)
* Home - Blog Style with Excerpts
* Home - Blog Style with Full Posts
* Page - Standard Width
* Page - Full Width
* Page - Contact Form
I also think that the default theme should be impressive looking and easy to read. Why NOT have it good enough that people actually use it? I'd model the look and feel after the excellent themes at ElegantThemes or StudioPress. You don't need to (and shouldn't) include all the plugins that they do, but something like the look and feel of those themes would be awesome for showing how good WP can look and how its way more than just a simple blogging system.
Oh, and some basic SEO features should be built in as well. I'm not saying to have a bunch of options, but simply implement best-practice for things like building the meta description... use category and tag descriptions and/or post excerpts, etc. where appropriate.