I'd like to share a concern I have on D7 UX. I loved the usability testing session at DrupalCon (great work guys!), but a big fat red flag popped up when it was mentioned that one member had a problem with the default behavior of orphaned nodes. A tester had a problem with newly created content "not being a part of the site." New content was created, but it wasn't added to a menu, or attached to site architecture in a meaningful way.
I love that a default Drupal install does not impose IA on a developer. If you want a search-driven IA like Wikipedia - you can have it. If you want a faceted tag-driven, IA, you can have it. There is progress being made in the realm of context in Drupal and we'll hopefully get to the point where we can cleverly build context-driven IA. And of course, there is always the option of a menu-driven IA.
Lack of a default IA impedes on usability, but I lean towards thinking it is the responsibility of the site developer or the install profile to establish IA and that this is not an issue to be tackled in Drupal core.