Nothing is a bigger turn off for me when reading any sort of material than improper grammar. It can make a company lose its credibility faster than the time it takes to hit the back button on a typo-ridden website. It takes a good design and turns it ugly, takes a thoughtful Facebook post and turns it into a laughing matter.
Here is one that didn’t make the cut but always trips me up: who/whom. If you are referring to the subject of a sentence, use “who.” If you are referring to the object of a sentence, use “whom.” Put another way, if you can substitute it with “he” or “she,” then use “who.” If you can substitute it with “him” or “her,” use “whom.”