My pet peeve is people using the word less when they should be using the word fewer.
Mostly this is a problem when people are speaking. Online I take it as given that spelling, often just mistyping, slightly off sentence structures and loose grammar are to be overlooked. It is part of giving the poster the benefit of the doubt. Of recognizing a common intention to engage in quick and easy communication, and focusing on the message being transmitted instead of the mechanics of how it is delivered.
When someone points out such errors online I immediately assume the person is either a prig taking a cheap shot to demean someone or someone who is being incredibly naive in their assumption such correction will be well received and helpful.
Professional communicators should know things like the less/fewer thing as part of their professional skill set and in their case sloppy grammar hints at sloppy professional standards. Working class stiffs get judged much less harshly.
That said the ability to speak and write clearly is a valuable skill. Sloppy speaking habits, like excessive insertion of 'um' and 'you know', rising valley girl inflection that makes every statement sound like a question, and mumbling can rob a person of their ability to focus and organize people in a crisis. Good, even if a somewhat less than perfect, grammar and clear, direct delivery gives a person an air of authority, and an edge in taking charge.
Something simple, like a shirt with a collar, can have people assuming your in charge. A hardhat and clipboard are so effective in conferring authority that they might be considered a disguise if your not actually endued with authority.