One I hate is excessive spelling and grammatical errors, aside intentional ones intended to make a point. I understand that those with poor English skills have a hard time with this and are learning. And that Germanized English is actually poking fun at the Watchtower Society comparing it to Hitler, so I can accept that as such.
Why? Spelling and grammatical errors, except when intended to accent a point, can make a post harder to read and understand. A few will not hurt, but if every other keystroke is a typo, it can become extremely hard to read and understand a post. We all make typos, and some will get past our best efforts to catch them. But, if you do make a typo and see it, it helps all who read that post if you find and use the backspace key. That will make posts easier to read.
This does not apply to intentional misspellings. Such as Craptower, Kingdumb Hell, Jehovah's Witlesses, a$$embly, and the like--those all help enhance the meaning. In fact, I actually think posts including those variances enhance the meaning of what you are saying: calling it "witless" means that Jehovah's Witnesses have no wits and are therefore witless. And the Kingdumb Hell is just that: it's dumb to go there, and it is hell. Those are helpful.
For those who are struggling with learning English: Keep practicing! That is the only way you will ever master it, and we will have to put up with it until then. This is especially important if people from Mexico, Brazil, and Nigeria are going to start posting. If you use software to translate your native language into English, that will help too--and we will accept whatever errors are inherent with such.
I do think the other format tools are good when properly used. One huge paragraph is not easy to read, and using all caps also makes it harder to read (though it is possible to accidentally hit the cap lock button and not notice it until it's too late). And judicious use of the forum features like large text, emoticons, colors, highlight, bold, strikethrough, and the like enhance the posts.
Notably, I have very few problems reading and understanding the posts on this forum, since most of them are reasonable and follow reasonable grammar and spelling.