If you have a point to make and want it to be respected and listened to you should at least have the courtesy to click on SpellCheck and make sure you sound as intelligent as you want others to believe you are.
A mistake every now and then is acceptable (because we all do it in a hurry) but when you write a long post with a lot of errors or continually post with spelling errors and terrible grammar, people will become annoyed. It's hard to take the point of someone seriously when they can't even take the 5 seconds to click Spellcheck.
I also think it's difficult to take someone seriously when they abbreviate everything. IM lingo, if you will. I can't stand trying to read someone's post when it is filled with it. Again, abbreviating here and there is fine (we all do it!). But all over the post in every post is just annoying.
But that's just my opinion.

What do I know?