I might take the opposite view. In the casual forums a more conversational style is appropriate, and one can overlook bad grammar and punctuation within reason. In the other forums, especially those where Members are asking and giving help, it is a different story. I think it is extremely rude and uncaring if someone posts about a problem in a way that makes clear and easy understanding difficult; Members who help do so in their spare time, and the easier one can make it for them, the better. Clear and distinct writing in Standard English eliminates confusion as well as lengthens the time for both parties if needless correspondence in a thread is required just to understand the problem or the solution. Now many of our Members read and write English as their second (or fourth, for that matter) language; they are taught Standard English. Imagine their consternation when trying to wade through a breadthless post without capitalisation or with slang and IM abbreviations; there are enough technical terms when working with computers that may require trips to the dictionary without their trying to figure out "b4."
It is not as if there is English and there is, say French. There are many different versions of English, ranging from Formal to Standard to various argots, and each has a different purpose and a different intended audience. Using Standard English seems appropriate when attempting to communicate successfully with a very wide range of people, just as IM English is appropriate when typing quickly and "talking" with friends who have a "feel" for what you are saying.
You are absolutely right John, and I suspect that some of our members that have learned English as a second language write and speak the language better than some of our members that are native to the language.
I have probably been perceived as being a nit picker about punctuation in these forums, but my concern goes beyond what is being written here, especially by our younger members. All of us have gone through phases growing up where we had phrases that were identifiable with our peer groups, and I suspect that this lack of punctuation and abbreviations are an extension of that same phenomena and hopefully will pass as well. How a person represents themselves in writing here is the only means that other members have to base their impressions on, and if you use short cuts, abbreviations, poor grammar, a lack of punctuation and capital letters it is going to be the indicator of what and who you are.
I also agree with DSTM regarding members that are adults and have huge numbers of post but won't bring themselves to set a good example to our younger members by using good communication skills.