1. Use improper grammar. I am a writer. I am a reader. I'm not a grammar fanatic (although some would say otherwise); but blatant or continuous mistakes drive me crazy. And guess what? If the top posts or most recent posts in your blog are riddled with problems, I'm not likely to click 'follow.' Just because less literary people accept errors they themselves make a lot, does not mean everyone appreciates trying to sort through your misplaced commas or run-on sentences.
2. Use improper spelling. This is very close to #1. They're both very common mistakes that the average person would overlook. I admit, sometimes I get too picky or the editor in me kicks in and all I can see is red. Most times, though, I can understand a few words out of a hundred. Your and you're; they're, their, and there; or too, two, and to are easy to mix up. When over a quarter of your post is misspelled, however, I am not a happy reader.
3. Have a slow blog. Dozens of advertisements, too many posts on one page, or a cluttered sidebar can all contribute to this problem. (So can photos, which is why I'm not much of a photo blogger.) Most blogs, in fact, are not slow enough to annoy me. When scrolling down your whole home page takes more than 30 seconds, however, you can be sure something's wrong. Unlike #2 and 3, though, this problem annoys more people than writers. So I suggest you see if it's fixable.
4. Make following hard. All bloggers want followers. It's a fact of life. Some blogs have two hundred and are shooting for five hundred. Others have a thousand and want fifteen hundred. (Others, like me, would go crazy over reaching fifty.) Keep in mind that readers who have to scroll to the bottom of your page or click on another link to follow are rather annoyed. Plus, it's a good idea to offer more than one mode of following. Different people prefer email, Google Friend Connect, Networked Blogs, an RSS feed, or other methods of following. If you allow for these preferences, you'll have more - and happier - followers.
5. Make contacting you hard. Some bloggers don't want people contacting them. Others want to remain anonymous. Some have absolutely no reason to have readers contacting them. This one mostly applies to book or review blogs, because a company who wants to contact you won't go to nearly as much trouble as I might. Sometimes, though, a reader just wants to ask a question or chat. As a rule of thumb, you should always have a contact page for these readers or companies.
What are your pet peeves - as a blogger, a reader, or another role you play in life?