I've never understood the programmers that write off design as fluffy stuff for the "creative" people who wear black turtle necks and berets. As if design was somehow beneath them, or as if it's wallpaper that you can plaster over a project once the code is written to "pretty it up".
To answer your questions:
Why are you interested in learning more about design?
Because I want to spend my career making people feel the way I felt when I first used an iPod. I picked the iPod here because it seems to be the stock example that gets trotted out whenever someone needs to reference good design, but you can replace it with whatever well designed product you like. We've all had those experiences in our lives where we use something new, and it makes something that was previously difficult easy. That's the kind of stuff I want to spend my time creating, and a sound knowledge of design is a means to that end.
What have you found confusing about design?
When and how to break the rules. I'm a fan of Robin Williams' Non-designers Design Book (http://www.amazon.com/Non-Designers-Design-Book-Robin-Willia...), but as a consequence my projects can look a little sterile because I follow the rules exactly as she lays them out.
In contrast, if you go and look at something made by Jason Santa Maria (an outstanding web designer at the top of his game) (http://jasonsantamaria.com/portfolio/) you'll see he breaks a lot of the rules, and it doesn't matter. In fact, it often times makes the work better.
Perhaps you can't codify or reverse engineer that. Jason has design talent. Me, not so much.
What is one blog post/book/movie, etc. that has helped you better understand something about design?
I'm fond of The Contemporist (http://www.contemporist.com/) for its great photography of great architecture. It's a great place to draw inspiration. My favourite post so far: http://www.contemporist.com/2010/05/20/chicken-point-cabin-b...
What software tools do you find useful when designing?
For HTML/CSS/PHP I tend to use Espresso (http://macrabbit.com/espresso/). I never mock up web pages in Photoshop. I go straight to code.
For desktop software, I'm a big fan of Microsoft Visual Studio's IDE.