User interface:
Drupal’s user interface has a steeper learning curve than that of WordPress. Users of WordPress are mostly bloggers, so the interface emphasizes a small set of the things bloggers need to do most frequently, like writing new posts, editing/deleting old posts and viewing statistics. The dashboard aggregates these high-use panels as a default landing page after authentication.
Users:
Drupal users have more diverse needs than just blogging support. Drupal provides more flexible layouts (panels) and information displays (views) than anything in the WordPress framework currently has to offer. An inexperienced, non-technical user would probably give up long before they got Drupal to do anything like what they wanted, whereas the default post-management behavior of WordPress caters to the goals of users who want to spend more time creating content than managing it.
I believe the fundamental difference between Drupal and WordPress is that Drupal fulfills the needs of developers while WordPress fulfills the needs of end-users. This means if you are developing WordPress plugins, you’re developing software for normal people, and if you’re building Drupal modules, you’re developing software to help people build websites for normal people. Not to say that people don’t hire WordPress developers to build custom sites, but if you want a site that features a non-blog-like information architecture or layout, a Drupal developer will be able to build your site cheaper by spending less time.
Extendability:
Drupal’s information architecture is more abstracted and more extendable than that of WordPress. In WordPress the post is analogous to a node, but no one has gone as far as to create a data-structure in WordPress that is as infinitely extendable as Drupal CCK. The closest WordPress has is a post metadata table, which really doesn’t come very close.