The basic commands: stuff like
wc. For those more familiar with Windows and MS-DOS,
wccounts the number of words in text files. Actually, you can use all these commands in Windows itself if you install the GNU CoreUtils package; this may have an earlier version of the CoreUtils than the one that comes standard with Linux, but the commands work just the same. GnuWin32 has a lot of Windows versions of Linux packages that you can install and use; they may not be updated anymore, but they can still be useful. I installed most of these packages in XP and used them a lot, though the last couple of years not as much as I once did.
Also, there's the package managers standard in Linux distributions. Ubuntu is based on Debian (and Linux Mint, which I installed on a partition on my new computer, is based in turn on Ubuntu), so it uses Debian's package manager, APT. The advantage of package managers is that they make it easier to keep your software updated. I missed that. I also missed the command line tools like
apt-get. When the XPocalypse finally gave me the chance, I plunged back in.
The desktop: naturally, it's got differences from Windows. For one thing, there's several you can choose from. I was a huge fan of KDE back when I had Kubuntu on my old and now defunct Gateway. Ubuntu Studio comes with XFCE; the version of Linux Mint I chose for the cute little 64-bit dual-core unit in my home theatre system uses the Cinnamon desktop that is just about as processor-intensive as the Unity desktop that comes with standard Ubuntu, or for that matter the heavy-duty desktops in Windows since Vista. In both my Linux distributions I had to assemble a few desktop components, especially some control panels that were missing. But learning the ins and outs of my chosen Linux desktops is the easy part.
The hard part is, as you'd expect, the deeper aspects of the command line, and the heavy-duty text editors I prefer but haven't been using lately, Vim and Emacs. For this, I'll have to explore deep into the jungles of documentation that surround them. NaNoWriMo is just around the corner, though, so I'll have to take my time with that.
One new thing I'll have to learn is how to use the advanced audio system called JACK (recursive acronym: JACK Audio Connection Kit). I know nothing about it. I need to find documentation and tutorials for it. I want to at least get competent in using it before FAWM, which is only 3½ months away.
Anyway, I'm happy I've reunited with Linux again. I even have Wine to run Windows programs again, and I've even installed a few games (Minesweeper, that pinball game, and Hover from the Windows 95 CD). I'm not starting from scratch, actually. Still, there's a lot of stuff I have to learn before Linux becomes as intuitive to me as Windows.