1. Posting excerpts of your own work, especially lengthy ones. Who really reads these? Best friends or your critique partners who have already read them? If you intend to publish these works someday, a future publisher may not want these floating around in cyberspace. The rule of thumb is to keep unpublished excerpts short, no more than 1-2 pages. If you're soon-to-be-published, make sure you get the okay from your publisher to post an excerpt from your upcoming book. Also, there's always the possibility that an unscrupulous writer may "borrow" your ideas.
2. Posting personal anecdotes and confessions. These are better suited for personal emails to close friends. Do you want an agent or editor going to your site and reading about your spat with your husband? Do we really need to know the nitty-gritties of your hysterectomy, or how your dog vomited on the laundry?
3. Stats of queries and rejections, posts of actual rejections. When you're published and famous, you can encourage others by saying how you've persevered through 187 queries/rejections and worked for 7 years to become published, but for beforehand, it's probably best to keep those to yourself. Stay general if you say you've been rejected! Imagine a future agent or editor reading what you've written; it may affect a future sale or business relationship.
4. Business relationship details. Don't go on about your breakup with an agent, publishing house, publicist, or critique group. If you have a disagreement with these people, discuss it with them directly--or vent in private to close friends.
5. Status of your shopped manuscript. When you're on submission, don't list the details or length of time you've been hunting for a publisher. You can undermine your agent's work. You can ruin a potential deal. An editor may Google you and discover you've been out for months and months--and they are last on the list. For sharing good news, usually a writer waits until a Publisher's Weekly announcement is made.
6. Contract or book sales details. This includes contract specifics, number of author copies, number of books you've sold, etc. There are confidentiality clauses in publisher contracts; be sure to get a publisher's permission before sharing anything.
7. Monetary details. Comparing your advance amount or royalties with other writers' can cause dissention or grief. Consider that a huge advance isn't always better, because it's more difficult to "earn out" and begin collecting royalties--and publishers are more wary to publish you again if you don't "earn out."
8. Generally whiny or disgruntled rants about waiting on agents, how unfair the writing world is, how long it takes to get published, the dreck editors are publishing nowadays, how you loathed another writer's book, etc. Do you want to encourage or inspire, or be seen as a complainer?
9. Politics and religion. People have strong feelings about these; you're asking for controversy. It may be fun to generate a rousing discussion, but if you're aiming for publication, remember that you could be alienating future readers, and that agents/editors may browse your blog.
10. Identifying information. Be careful posting photos of your children, specifics about vacation plans (travel itinerary), info about where you work or live, your telephone number or personal email (it's recommended to get a separate email to use for blogging). Don't advertise to a burglar that your family will be away for two weeks!
11. Responding to negative reviews of your book. Don't. Do. It. It generates bad public relations, and doesn't cast you in a favorable light. The backlash is usually severe.
12. Constantly marketing your own book. Once we've seen it, we've seen it. Your followers already know the info, even if a few new followers may not. You're not increasing your sales or outreach.
13. Gross or bathroom humor, off-color videos, swearing, etc. Is this professional? Do we want an agent/editor to see it? We are writers--surely we can use our imaginations and find alternative ways to say things to avoid being offensive to some people.
14. Complaints about your editor's recent revision notes. No matter how you despise your editor's edits, remember that he/she might read your post!
15. Bragging. Be humble. No writer "has it made," and shouldn't treat followers as obsessively adoring fans. We are ALL learning, and we're at different points in our writing journeys.
Be careful out there.
Do you disagree with any of these points?
Can you add anything else to this list of things NOT to blog about?
Have you done one of these things, and regretted it? Will you change anything?