Honestly, I wish I understood the filesystem better.
I came from the Windows world with very little programming experience. Even though I've used Ubuntu as my other OS since 10.04 (and experimented before then), I still don't understand the filesystem beneath it. Everything is in something like /bin, /etc, /lib /home, or some other very short folder. I could at least make the connection that /home is for my stuff, because that's where all my stuff is saved by default. But just about everything else is a mystery.
When I install a program, where does it go? In Windows, I've been trained to look in the "Programs" folder. In Linux, I have no idea where it went. Did it go into /bin or /lib? Or somewhere else? What are those separate directories for? Why is the .conf in a different place than the program? If I want to install something myself before it hits the distro's repository, where do I put it?
What Ubuntu lacks (along with just about every other distro aiming to convert Windows users) is some built-in way to teach users how to do the things they used to do in Windows. I don't mean things like "how to open a Word doc" -- if you can't find LibreOffice in the menu or use an online doc-editing tool, Linux is beyond you. But advanced users of Windows could use the boost to get them on the same level as beginner-to-normal Linux users.
I have thought of writing some kind of book or website tutorial on just this topic several times, but I can't -- I don't know the answers to these questions. If someone or some group did this, that would help make Ubuntu a more mainstream desktop OS.