In Canada, this is something we do very often. It depends on the context; if you're talking to your boss, maybe don't. But when talking to friends or strangers even I do it quite often. It makes you look friendly!

I would say it's completely dependent on context. I tend to use a lot of exclamation marks, but only when I'm excited about something. Sorry it can't be more clear cut, but I wouldn't read too much into a simple overuse of the exclamation mark. Hope that helps.

RUDE I can't believe you didn't say hello! You must think you are better than me! NOT RUDE Sorry, you didn't say hello. I guess you didn't see me.

At work it should be seldom used unless it's about something exciting. Even when used then it is informal.

It is a little bit "off putting" to say the least. Depending with whom you're corresponding, emails tend to be more formal. Adding an exclamation mark after every sentence would seem silly and a bit unprofessional, especially in a business context. However, with friends I'd say it's a bit different. Think of it like text messaging I hope that helps