I've used Ubuntu for ages (Love it!). A few useful things I wish someone had told me a while ago:
WARING: LOTS OF TEXT AHEAD. You were warned.
Like others have said, get used to bash. If you really want to understand Linux and how it works, avoid the fancy graphical stuff as much as possible. Compiz != Linux
Press <TAB> to auto complete many things in the terminal. There's a lot more keyboard shortcuts than that, but <TAB> is probably the most necessary. It can complete file names, commands, application names, even some arguments (If there's multiple options, <TAB> twice should show them all). You can even use it for apt (sudo apt-get install intelli<TAB> will extend "intelli" to "intellij-idea-ce", the full name of the IntelliJ package. "ecli" expands to eclipse, "emacs" followed by two tabs will show all packages starting with "emacs"). Saves you having to search for packages using apt-cache search <packagename> .
Learn all the basic commands (cd, ls, rm, mv, mkdir, cat, killall, less, ...) and also how to combine commands via piping. It's a bit advanced, but pretty useful.
In the default desktop environment, ctrl-alt-t opens a terminal. Much quicker then clicking on something, and if you're like me (and practically every other Linux user), opening a terminal is something you'll be doing a lot.
Ctrl-alt-f(1-6) changes tty shell thingy. If ever you encounter an issue with graphics drivers or something goes wrong and you can't fix it graphically (e.g. can't even open a terminal). Ctrl-alt-f7 will take you back to the graphical session.
Often, you'll want to install something that isn't available on Ubuntu's repository (aka you can't get it with apt-get). You might be able to find a repo and install it that way, but IMO that whole process is a total PITA. For most Linux projects the website will have a binary, but often you'll have to build it yourself. Download the source, install 'build-essential' if you haven't already, then try 'cd'-ing into the directory and running './make'. if there's a file called configure.sh you should run that before make, and if there's a README then obviously read it. It seems really complex the first few times, but it'll become second nature pretty fast. Also note often programs will use libraries, which you'll also have to download. Generally though, 'apt-get install' can get the libraries. If something asks for 'libfoobar', there'll probably be a package called 'libfoobar-dev' available. Sometimes it can be a bit of a pain to build things though. When the building gets tough, remember:
Google (Or DuckDuckGo, or Bing, or whatever you use) is your friend. Most of the time. People on online forums are apparently friendly too, although I'm not the biggest forum fan. IRC channels have helpful people on them, too. Although some people get seriously mad if you type their name on IRC. I don't know why.
If ever you need to edit files from the command line, nano can generally do the job, and it's really easy to use. Of course, if you prefer Emacs/Vim, then go with that, but I'm not suggesting you learn either of those.
Also note there's a project called 'wine' which lets you run Windows stuff on Linux, and there's virtual machines available on Linux that you could install windows on. I'd suggest you avoid those though, unless there's a program you can't live without. Most popular Windows programs have a Linux port or equivalent, though.
Even though people have said Linux isn't very good for games, games written in Java still run fine (And, really, why would you play games written in anything else?
). There's even a version of Steam available for Linux. You'll probably have to go without your CoDs and your Battlefields, but more and more developers seem to be recognising Linux as a platform, possibly due to the Humble Indie Bundle (Which is awesome, check it out if you haven't already). WINE can play a decent amount of Windows games, and emulators for most older consoles are available.
I just wrote a massive wall of text. Not certain if I should be proud of that, or if I'm just rambling. I wish it was this easy to write essays...
EDIT: As for the fan issue (You posted that while I was typing this so I didn't notice
), I had it too.
Short answer: It's a TOTAL pain in the rear-end. Really. Unless you're issue is different to mine.
I have two GPUs, a GeForce 540 and a Intel one. My issue was that my computer was using my super-powered GeForce always, even when it could have used the internal one. Apparently, NVidia fixed this issue with the latest version of their drivers, so try that (nvidia-current I believe it is. Don't use the mesa driver). Otherwise, search up on a project called 'bumblebee'. It might work out of the box for you, or you might spend a few months trying to get it to work. The one and only issue I've had with Ubuntu was my GPU.