On the former blog I wrote I used to give my opinions on everything from politics to celebrities to religion. And I found that for me, that is a big no no.
Talk about getting slammed! I learned through that experience to keep my mouth shut. No one needs to know all those things about you nor do they want to. Relate to people on the topic of your blog and leave it to that if you want readership and a following (unless of course, your blog is about one of those things specifically).
Not sticking to the first blog I ever started, it's the only blog-moment I regret.
Spending way too much time using Blogger.com as my platform before switching to WordPress. It's a pain to learn at first, but worth it ladies.