What's the worst thing that could happen to a blog — or anything else that you were doing with learners and technology?
The worst that I can think of is for the work done all to be lost, perhaps through some failure of the (often free) Web 2.0 service you might be using.
On the Blogs and Blogging course I was teaching in July, I thought that had happened as (right in the middle of the course!) Blogger's automated spam filters suddenly decided to block access to the blog we were using, deciding that we were spammers, possibly (and it's the only reason I can think of) because of the high incidence of the word "blogs". Fortunately, access was restored to it very quickly (though the same thing has since happened to another blog I was using on another course).
One of the problems with free web services (especially small companies) is that they get bought out, or go under, and there is a risk of the worst happening: your students work being lost. Even with services that do survive, things can go wrong (I had a major problem with Podomatic "losing" a lot of audio archives my students had created, a couple of years ago now).
Whatever you do with technology, make back-ups!