Formality is certainly much less common than it used to be, and most business communication is about speed rather than etiquette. However, there are still some situations where using correct forms and formal style would be strongly recommended. For most jobs, a letter of application is expected to start and finish with a recognised form of address (eg, "Dear Mr Brown" unless the advertisement says "Send CV to Tom") and to be written in a polite, serious style avoiding slang and short forms. A letter of condolence, even to someone you know quite well, should be correct and serious in style – and should be a letter, not an email. People often keep these letters and reread them – they can be very comforting. A complaint should be written as formally as possible. Jokes and slang will do no good at all – you need to be coldly polite, but make it clear you mean business.