For fanfiction, at least, sometimes the Wall'O'Text isn't necessarily the author's fault. I've imported stories from one place to another and all the formatting was borked horribly. It took me a few weeks to actually figure out how to fix it, and in the meantime it was just a huge mess of text.
Not that it's the case all the time, but sometimes you just never know.
One of my biggest things is people who don't know how to use elipses. I've actually seen someone string a bunch of commas together to try to indicate a really long pause,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,like this. No. Just...no.
Funny story driving today. I was on the highway and was grading the traffic patterns with punctuation marks. This slow down was a semicolon, that one was a colon, they were both close to full stops but not quite there. This was a comma, it was just a slight pause in the flow of traffic. It was fun.