What is main difference between spoken English and its written variety

I think there are several differences between written and spoken English.  To categorize it, I suppose it could be said that written English is more formal (usually) and spoken English allows for accent and dialect differences.

Here is an example of an acceptable spoken sentence:

The musician didn't want their song to be released until the end of June in order to boost sales just before the big tour.

However - in writing this would be wrong.  The correct way to write it is: The musician didn't want his (or her) song released until...

Note: One singular musician cannot be represented by the plural possessive pronoun "their" - but MLA has said this is acceptable when speaking.  We do it all the time because using "his or her" verbally is muddy - also when speaking we often want to keep a person's identity hidden so we use "their" instead of revealing whether we're talking about a male or female.

Other accent/dialect examples:

Y'all in the south = you guys in the north.  Waiters and waitresses swear though, that referring to a table of both men and women as "you guys" often results in a lower tip.  On the other hand, not many people say, you all.  Again - slow, awkward, muddy.  And certainly it isn't proper English to write y'all.  In fact, my southern friends do it on Facebook all the time and spell it wrong: ya'll - making it worse: "ya all."  Aweful.