This thread is intended to be a reference (hopefully sticky, wink wink) both for new users with common questions and for lazy Q&A veterans who want a quick link to an answer for a question that’s been asked before 
The chances are large that I’ve missed many common questions, reply with new questions or corrections and I’ll add them to the list.
“Import Site” warning message when starting blender: 1 2 3 4 5
Blender comes with a cut-down version of Python, for scripting. A lot of scripts run fine with this version, but some require the full load. The message is warning you that you don’t have the full python installed, and it’s using the built-in version alone.
Either install Python or alternatively just ignore the message.
S then X to mirror: 1 2 3
In short, this has been replaced as of 2.30 with Ctrl+M. S,X/Y/Z is now axis-constrained scaling, similar to G,X/Y/Z and R,X/Y/Z
Undo: 1 2 3
U will undo but only in mesh edit mode (so far!) Anywhere else, save frequently, and save a backup before doing any major changes.
Also look at the next question.
Automatic Temporary Saves: 1 (not asked so much on elYsiun, but often in #blenderchat)
Fortunately, Blender has an automatic temporary save. Also fortunately it is by default on, saving every 4 minutes (IIRC). Unfortunately, it’s default save location is /tmp which doesn’t exist on windows systems (I’ve heard that it will save to C:\Windows\Temp but this never happened to me).
In the user preferences (see next question) in the File Faths section set the Temp folder to one that exists on your computer, and in the Auto Save section make sure Auto Save Temp Files is turned on.
User Preferences or My default/new scene is not blank: 1 2 3 4
The user preferences are in a window above the main windows, drag down the frame just under the main menu to access it. Many settings lie therein.
To save them, press Ctrl+U but be warned: this saves everything in the blend file as the defaults, including any models you have (which can be a good thing, it lets you set up things like default lighting/camera arrangements)
If you have lots of garbage in your user defaults and just want to reset them entirely, search your computer for the file .B.blend (It’s either in blender install folder.blender or C:\Documents and Settings</b>username\Blender Foundation.blender on windows, or ~/.blender in linux) and delete it - blender will use its internal defaults.
No Actor Button in the game logic: 1 2 3 4 5 6 7 8
Change Physics from None to Sumo in the world buttons (F8) and the button should appear.
Hmm, that’s enough for me right now