Adaptivetheme is a starter theme for designers and front end developers - its an ideal starting point for CSS only themes and for themes that give end users control over the layout and design.
There are many theme settings that control the layout and what is displayed on pages and nodes, such as taxonomy term display, links, submitted by information, breadcrumbs, search forms and results and much much more.
The HTML is focused on web standards and semantics to improve the output of Drupal and contrib modules. The theme also includes settings for adding and removing CSS classes. This helps de-clutter the output and gives you cleaner, more manageable code.
Adaptivetheme is content-source-ordered for SEO and accessibility, includes skip navigation, other accessibility enhancements, and is RTL ready.
6.x-1.x to 6.x-2.x - remove the old theme completely, do not attempt to overwrite the 1.x version. You will need to port your sub-theme manually.
6.x-2.0 - 6.x-2.2 - remove the core theme and replace with the newer version. Several files were removed from the 2.0 version so you will get some errors from Drupal - just clear the theme registry. There were changes to both Views templates and content-field.tpl.php, if these don't suit your theme you can override them in your sub-theme with the original templates from Views and CCK.