What did you inititally struggle with on Arduino?

I'm thinking about making a few Arduino guides. Just wondering what you guys got stuck on when you first started.

Thanks!

Choosing the right serial port. :) :D

Board selection in the IDE caught me at the beginning. And Serial port selection, using the same kind of Arduino but a different PCB. i.e. Two UNOs.

Some great work done here: http://www.gammon.com.au/forum/?id=12153

When I got my first one I wasn’t sure what a bootloader was for. I also wasn’t sure if you had to plug something into the power jack (as well as the USB). I think I was also unsure what Vin was, and if it was important.

I also remember wondering which pins did what. Apart from digital input/output I knew some pins had dedicated uses (eg. SPI, I2C) but wasn’t all that sure what all those dedicated uses where. A page summarizing that would be helpful. I tried somewhat with this page: Gammon Forum : Electronics : Microprocessors : Arduino Uno Rev3 pinouts photo

I think I also found puzzling the advice that you “don’t connect a motor to the Arduino”. I remember wondering “what the hell do I connect it to then?”.

I think it would be helpful to show (maybe with diagrams) that things like a stepper motor can be controlled by the Arduino but not powered by it.