There are three circumstances when using an abbreviation is appropriate:
The rest of the time, do your reader a favor and write the whole word.
Here's a little quote taken from my restaurant days (when the wait staff often needed to use abbreviations when taking orders for the chefs):
If in doubt, write it out.
I agree that a writer does him/herself a disfavour by assuming the reader will immediately understand an abbreviation. Of course, I can say to fellow Aussie Lauren, "Up for a bevvy this arvo?" knowing that she already understands bevvy to mean beverage (ie. a beer) and arvo to mean afternoon, but I wouldn't use that phrase with any English speaker, native or not.
In addition, I sometimes get thrown when my Filipino friends use terms like TMI, PDA and CR ("too much information", "public display of affection" and "comfort room", ie. toilet/bathroom) when they speak English. I need a moment to interpret what I actually heard. So, in that respect it still depends on who you are talking or writing to.
That said, I still agree with your guidelines for abbreviations.
I disagree. I think the preference for abbreviations or against them is one that is largely based on dialect. Standard Australian English for example is perhaps more prone to abbreviating words and is thus more accepting of them - any google search of Australian slang will demonstrate this. Furthermore, if characteristics of dialects are ignored, often the use of abbreviations is situational or based on context. For example when it comes to contractions, to me, "I'll see you later" is far more natural than "I will see you later" and anyone using the latter would be easily identified as a non-native speaker. Likewise for things like "gonna, wanna, etc." the use of which is often far more natural when it comes to emulating native speech patterns.
here is one more circumstance
4) when swearing in public
i am sure you dont wanna see examples :D
I'm including the impromptu shorthand some use composing a text message with their thumb while driving in traffic. I see a lot of that online, used just as if it was legitimate English. It's easy to forget the fact that the purpose of language is to communicate with others. It behooves us to make life easy for our reader when possible, if only as a matter of respect. "They'll figure it out" is not the right way of looking at it. This is especially true when posting in a public forum. That said, restaurant jargon is a world of its own.
Cursing is a subject unto itself. I've noticed a number of people here take an interest in it. It's an important part of English that is often overlooked, and it can be done well or poorly. It would be a good subject for a discussion of its own. A "Cursing Workshop" might be popular. :-)
I disagree with Laura, definitely prefer Eddie point of view. He wrote even a topic about such "barbarization of the language" (this is my personal opinion, not His). Sorry, Laura, but teaching people slang, especially from such distant place as Australia is not a good idea. Maybe for some youngsters from Asia, definitely not for people from Europe, Americas, Africa.
Learn apprioprate language first, then go for the slang.