seeking email (increasingly common these days) is unlikely to guarantee success, whist including half-formed thoughts in an exam situation has tended not to be recommended as a strategy.
As for commenting, I would suggest that bloggers should make it both possible and easy for people to comment on their blogs, in order to encourage the online conversation. Unfortunately, though, some people’s conduct is such that it’s necessary to either moderate comments before they go live, or have some kind of registration system that people have to go through before being permitted to make comments (though this isn’t foolproof).
The corollary of this, of course, is to ask would-be commenters to make comments which are both polite and useful. For example, simply saying “You are wrong”, is a pretty pointless exercise because it does nothing to move the argument on.
A good rule of thumb, not just for blogging but for any online activity, is: if it’s not acceptable in the physical world, then it’s not acceptable in the virtual one.
http://www.bellaonline.com/articles/art1204.asp. http://weblogtoolscollection.com/archives/2004/08/02/blog-comment-etiquette/. http://blogs.tedneward.com/2005/08/27/Comment+Etiquette.aspx.