Peter Westwood writes about two significant changes to the WordPress code that affect WordPress theme development:
The get_template_part template tag
The first is a new template tag and feature called get_template_part, which helps recycle oft-used blocks of code like the WordPress loop.
get_template_part takes in two parameters: the first is the name of the generic template [...]

There’s a popular Reddit thread that laments the ambiguous labels used in the pagination links, notably on WordPress:

To me, clicking on ‘next page’ should take me to the next page. Next comes after. Next means later posts, newer stuff, and so on. The ‘previous page’ link should go to the god damned previous posts. It’s [...]

JavaScript is an important part of creating a WordPress theme, and the jQuery is one of the most popular frameworks already integrated into the WP core, so one basic technique theme developers should know about is making a theme dependent on jQuery play nicely with how WordPress loads it. This crash course should help.

Use wp_enqueue_script [...]

op111.net explains how WordPress Child Themes work and what you need to make your own. The crucial line that separates ordinary themes from child themes it the Template declaration in the stylesheet: Template: Kubrick means you’re basing the child theme on the Kubrick theme.
The tutorial even touches Firebug usage and explains what several CSS rules [...]

Otto on WordPress has compiled a list of the absolute minimum requirements every WordPress theme should have:

Too often I see themes missing the absolute minimum requirements to make the theme actually work properly. So I figured I’d make a list of things that ALL WordPress themes need to have in them, every time. These are [...]

Here’s a neat discovery by Andrew Ozz: you can actually style the content of the WordPress WYSIWYG editor so that it displays how text and images will actually look like once posts are published. Matt Thomas, who leads the development for the new default WordPress “2010″, calls it WYSIWYRG (what you see is what you [...]

BloggingPro has a great rundown of the newest theme files introduced in the latest version of WordPress. Here’s the short list:

The category-slug.php template: Before, theme authors were limited to using category dependent theme files that used the category ID. In WP 2.9, category slugs can now be used in the category ID’s place. That’s a [...]

Two beta versions of WordPress 2.9 are out in the wild, so you know the final release will be announced soon. Have you looked at the newest features in WP 2.9 yet? If not, check out Technosailor’s round-up. The big news for WordPress theme authors is that post images are now integrated into the core. [...]

It’s official, WordPress themes are GPL too. Matt Mullenweg announced at the Development Blog that after consulting with the Software Freedom Law Center, it was determined that “PHP in WordPress themes must be GPL” although “artwork and CSS may be but are not required”. This means that WP theme code can be modified and distributed [...]

The latest version of WordPress, version 2.8 and codename “Baker”, is now out. If you haven’t updated yet, you might want to do so now. One of the biggest features in this version is the ability to install WordPress themes from within the admin dashboard, just like plugins.
Theme authors will also be happy to [...]