Do follow blogs are helping to increase traffics to your blog, because they allow you to comment or leave a link to your site.
But it also can make your site’s reputation going down if those links are a bit spammy.
WordPress blog by default has a no follow attributes. It is made to protect your blog from spammy or junk links and comments.
Here are my own experiences with this blog.
1. I installed commentluv plugins to let my commenters linking to their posts. This plugins is really useful if you can really be careful to pick which comments are spammy and which are not.
I end to remove this plugin, because i am not good to filter the unrelated comments, i am too nice ;)
2. Having unrelated, junk, and spammy links in your sites, are going to hurt your site’s reputations, and will hurt your page rank too (if you care about it).
So my tips are, if you let your blog to be a do-follow blog, you have to be extra careful and watch out for the links inside your blog, as a newbie, i used to be really happy when someone is commenting to my site.
One thing you can do, if you want to approve a comment and not the link, just delete the link, and approve the comment.
Here is a good video from Google webmaster why do-follow blogs can hurt your wordpress blog if you are not careful with your comments or links in your site.