It depends on the learner’s previous experience. Someone who used to build sites with traditional HTML has to understand that there are essentially three different systems to control: the main content area (via nodes), surrounding areas (via blocks), and general look of the site (theme). Someone who’s only had a MySpace page could be overwhelmed by Drupal’s openness.
The biggest hurdle for everybody, I think, is installation. There’s only so much a Drupal instructor can do about that: The problems are related to how the Internet works. Getting a domain name, selecting a web host, moving a site from your local machine to a server… these are all very difficult tasks for someone without previous knowledge. Acquia in particular has done a lot to make these tasks easier with its Drupal Stack Installer and hosting services; other companies have done their part, too. But it’s still a mess, and will be for a while.