Here is a round-up of things that have me clicking elsewhere before you can say, well, anything actually:
Captcha - this is the deeply irritating thing that forces you to copy some squiggly numbers and letters to "prove you are human". I find swearing and giving up also prove I'm human.
Requiring log ins - Some blogs are set up to make you to sign up or sign in in order to comment. Also very, very annoying. Sure, it's better to make a comment using an identity, but it should be compulsory.
Out of date content - If I find a business blog - or any blog - that I like the look of or am interested in, it's crushing to then find that there's been nothing fresh for months.
Bursting posts.Clearly it's your blog, put what you like in it, but all too often there's far too much in one post. If your post is looking quite lengthy, ask yourself if you could chop it up into several smaller ones.
Big lumps of text. We readers are simply creatures who don't want to have to work too hard. Therefore make it easy for us to consume your delicious content. Use short paragraphs, have a line break between each paragraph, use sub headings and bullet points.
Follow me, follow you. Have some way that I can follow your blog without having to visit every day. Is there an email sign up, an RSS feed? I like you, but I've got better things to do than stalk you.
Don't nag. We all know that ultimately a business blog is there to help the business. But there's no need to shout that in our faces. We'll follow, buy, log on or share if we want to so back off and let good content speak for itself.
Explain things. It's helpful to assume that each post is a standalone thing and that whoever is reading it knows next to nothing about you. It might not be the case, but it means you'll give all the necessary information, introduce the key players and tell the back story.
Now you know what not to do. You can make sure your blog doesn't join the league of irritating, the ranks of the annoying or the army of unread.