Arduino Basics - What do you need to know?

So a couple weeks back I said to myself "I should write an eBook about Arduino programming!" Educating future Arduino users is something i've wanted to do since I started with Arduino so this seemed like a perfect topic.

My question is this:
What do you want to know?

What confuses you about the IDE, the hardware, the programming? Anything you've struggled with or anything a student is struggling with, please let me know!

Review past noob questions, lots of ideas there.