Dear Sirs and Mesdames,

What is wrong with writing formal e-mails?

I change the form of address depending on the audience.

That Mesdames is the plural of Madam was something I learnt a week or so ago, because I had to write a formal, general letter to our customers, translating from the German into English...

Nothing's wrong with it. It just shouldn't be *required* by the SPAG/etiquette police.

It depends, if the email has a formal tone I'll start it with "Dear Mr X", or "Dear all". At work I have to email a group of people daily to inform them of the work I've carried out, I usually start with "Morning all", or whatever time of day it currently is.
Informal emails start with "Hi", "Hay", or "Hullo" (though rarely "Hello").

If I have a quick fire reply to another email, I may miss out a salutation altogether...

I've never started an e-mail to colleagues with "Dear", infact I don't think I've ever started an email with "Dear". Similarly, I've never put the date and addresses in the corners.

I think it would be weird to write Dear to colleagues - you wouldn't walk over to their desk and begin the conversation with a "Good afternoon sir, allow me to introduce myself" would you?

When replying to an email - I will only say hi in the first response. If firing emails back and forth it's generally just like IRC or MSN - straight to the point, no hi or goodbye.

If you were composing a letter to, say, the local court would you format it with your address in the top right, theirs on the left, the date and a reference number, or just go for 'Hi'?
If you're writing a letter, or email, of complaint, do you start with 'Hi' as well?

I've always had an issue with dear for letters. Apart from the fact that it is not a word I would use face to face for even those who are dear to me, I find it a very odd word to use for someone i've never met.

No, and no.

I think this an issue of people who feel their superior understanding and/or use of their prized language makes them better than others.