An important part of the theme development process is testing. As a member of the Theme Team at Automattic I can say that we like to test everything we can! One thing that we have observed is that widget testing can take up a lot of time. WordPress provides 13 widgets, many of which contain a form enabling us to customize each instance. Populating a sidebar with widgets can be rather time consuming especially if you have to tweak each widget’s settings.
Man, theme options. 
There aren’t many more topics in WordPress theme development that inspire more discussion these days than theme options. If you haven’t seen it yet, check out Konstantin Kovshenin’s Theme Options Gallery. Konstantin reviews the best and worst theme options pages around there. It’s where the best discussion on theme options is happening right now.
If you’re interested in HTML Semantics and are looking for a good place to start learning more about it and what it means start out with this post at HTML5 Doctor, Let’s Talk about Semantics. At the very least, you’ll learn why we don’t have a pineapple element in HTML.
If you’re interested in learning more about how to use get_template_part() in your themes and why you might want to, designer and developer, Terry Sutton, has a great post up called Tighter and leaner WordPress templates with get_template_part(). It’s well worth a read. (And not just because he mentions _s though that’s pretty cool too.)
There are a couple of spots that I always keep handy when looking for information about WordPress’ query handling. Consider this Gist an addition to my list:
WordPress Query Comprehensive Reference
Which helpful, recent resources do you use for information about WordPress’ query? I prefer to hit the Codex (WP_Query and query_posts) and also just dig directly into wp-includes/query.php.
Fun fact of the day: about 37% of WordPress downloads are for non-English, localized versions.
So as a plugin or theme author, you should be thinking of localization and internationalization (L10N and I18N) as pretty much a fact of life by this point.
An excellent post from Otto on improvements to make and pitfalls to look out for when performing i18n on your WordPress theme.
There are some simple instructions in the theme readme.txt on how to get started with our new starter theme, _s, but, to be honest, I’m not exactly the most reliable readme reader myself so here are those instructions with a bit more explanation.
The first thing you want to do is copy the _s directory and change the name to something else. Like, say, megatherium.
Then you’ll need to do a three-step find and replace with your favorite text editor on the _s name in all the templates.
My coworkers at Automattic and I frequently discuss the speed with which we’re able to onboard new themes into the WordPress.com theme directory.
Our top priority as the Theme Team is to make sure that all of our users feel like they have a theme that fits them perfectly; in order to meet that goal we’re focused on bringing a variety of themes into WordPress.com through a few primary channels: the WordPress.org theme directory; premium theme shops; and Automattic (in-house) themes.
It’s often the case that each conversion—that is, making a theme’s code WordPress.com-safe and ready—will take us anywhere between one week and one month, depending on the complexity and quality of the code. In a perfect world, though, we’d be able to snap our fingers and have every single awesome-looking theme available on WP.com right now.
There’s a nice, clean, javascript-free, approach to mobile-first design in Joni Korpi’s post Leaving Old Internet Explorer Behind. Adopting it sounds pretty tempting.
If you haven’t yet you should make some time to read Building Twitter Bootstrap at A List Apart. Especially if you’re interested in building a WordPress Theme Framework or WordPress Starter Theme.
Twitter Bootstrap is essentially a collection of HTML-CSS templates and some Javascript put together to “help designers and developers quickly and efficiently build awesome stuff online.” It sounds an awful lot like a lot of WordPress Theme Frameworks and Starter Themes, right? It looks like one too.