Here's a few more. They might just be my personal pet peeves, but I think that they have merit.
8. Not using any capital letters (alternatively, only capital letters) or punctuation, and people who use "l334" speak. If what you have to say is worth reading, it's worth spending an extra 5 seconds to type it correctly.
9. Colors. Changing color is useful if you want to black something out to avoid spoilers or info about a duel. It's not aesthetically pleasing, it's not cool looking, and it's not going to make you stand out (at least, not in the way you want it too).
10. Needless text formatting. Making something bold and/or italic is useful when you need to make something visually stand out from the rest of the text. It doesn't make your point any stronger. Same goes for using huge letters.
Do these get on anyone else's nerves at all? The color one in particular really gets to me. I hate the way some people deal with color on the internet. Enworld is the only site I know that managed to get light text on a dark background and have it be readable enough for me to frequent it. Then again, I tend to dislike colors in general (I'm partially colorblind).