Formal and informal

Which phrase or type of language would you find in a formal letter? Which phrase or type of language would you find in an informal letter?

I am sorry to inform you that...

I am very grateful for...

Why don't we...

I will not be able to attend the...

exaggerated punctuation, such as "That's awesome!!!"

idioms and slang

contracted verb forms like we've, I'm, etc.

Give my regards to...

I look forward to hearing from you...

Let me know as soon as...

The use of emoticons, such as :) ;) :P

Dear Tom,

Dear Ms. Smithers,

Best wishes,

Yours faithfully,

common misspellings, such as "l8tr"

I'm really sorry I...

Unfortunately, we will have to postpone...

polite phrases

Love,

Use these websites as a resource to familiarize yourself with popular Internet slang
http://en.wiktionary.org/wiki/Appendix:Internet_slang
http://www.noslang.com/index.php
http://www.internetslang.com/
http://www.ruf.rice.edu/~kemmer/Words04/usage/slang_internet.html
http://en.wikipedia.org/wiki/List_of_common_emoticons