* Not linking prominently to their main site. (rdl is right on the money.)
* Using Google AdSense or similar programs on their blog. Sending away prospective customers for pennies is far from a good plan.
* No calls to action. Insightful articles are nice, but you need to lead the reader to take action, whether it's signing up for your newsletter, RSS feed, checking out your main site, a particular product, simply get in touch to say hi, or something else.
* Targeting the wrong audience. If you are blogging for customer acquisition, you need to answer the questions your customer base has. Blog about the information they are after. BTW, your audience is not everybody. Your audience is a specific, narrow group of people.
* Failing to promote their posts. Build it and they will come doesn't work in the blogging world.