Why the password "this is fun" is 10 times more secure than "J4fS!2"
The danger with semantically meaningful passphrases is that they have a <i>lot</i> less entropy than you think they do.  I still use them for everything, but it is something to be aware of.  (You can get more by e.g. padding it with a number, doing the usual l33tspeak tricks, etc.)