I've noticed what I think are becoming clearer segregators between the Drupal, Habari, and WordPress platforms; some details that differentiate the products more in my mind than they had before.
Drupal is constructed to let users build sites entirely from its UI. Much effort is made to make it unnecessary even to write any HTML code, yet still allow users to configure the type and quantity of data fields assigned to their content. Users can set options to determine the layout and formatting of output.
As a result of Drupal's user-configurability though, any little things that fall outside of what the system can do for you are often crazily more complex to apply than other systems. For example, if you want to apply any special formatting to a field of data that is entered on a node, you must write theme functions or alter a template's output of that field. This is often a more complex task than what would be required to output the same data in other systems, which is explicitly what Drupal is trying to avoid in the first place.
WordPress is constructed to have a fairly fixed set of data, in comparison to Drupal's field flexibility. By doing so, it can confine the user to a very strict set of data to output via a finite set of specific PHP functions - aka "Template Tags". This is a benefit to people producing WordPress themes because it gives them the flexibility to design HTML and CSS code, and then insert the functions for data output into predefined spaces in their templates.
WordPress is different from Drupal in that it assumes you will want to edit the template code to change the output, rather than configure the organization and display of data from inside the software. As a result, WordPress' code must stay simple to appeal to users who wouldn't otherwise grasp database design, and so would likely never have the level of data configurability that Drupal offers.
Habari is constructed to have a reasonably fixed set of data, more like WordPress than Drupal. A key difference though, is that Habari doesn't confine itself to simple tags for the benefit of non-coders. By leveraging an assumption of some coding ability, or at worst, the ability to cut and paste existing working code snippets that produce effective output, more dynamic data can be extruded and manipulated from the initial basic offerings.
Unlike Drupal, Habari is not designed to allow users to change the layout or presentation of the data on the page. Like WordPress, Habari assumes that a theme designer will want more control over the layout of output than could be provided by a generic UI. Still, in many cases, Habari requires that theme developers have a more developed coding skill to make use of the tools that it provides.