Presenting rumor, conjecture, opinion and fiction as fact.  If you're online, you can easily research what you're talking about, so maybe you should.  Include links to sources and information that support and expand on what you're saying.

Don't ramble on and on.  Keep your writing brief, concise and to the point.

Write in a conversational way, as if you were speaking to someone.  Too formal is boring in the blogosphere.

Go for quality over quantity.  You don't get extra credit for posting more often, so try to above all, keep it meaningful if you want to be read.

To me, the biggest mistake bloggers can make is not keeping their content up to date and relevant to their audience's needs.  I truly believe that content is king.  Anyone who regularly uses the web and reads blogs does so because they serve a purpose.  Perhaps they answer questions that the readers have, or they provide advice or "how to" tips, whatever the content, it has to be useful or nobody would bother reading it.  And if a blogger hopes to establlish a following the posts better be frequent as well. 

You have to make readers want to come back and read more.  It's all about what you offer.  If you rarely update your posts or your content is stale and uninteresting, don't expect to be successful in the world of blogging.