I know that for me, I get much more spam comments than actual relevant ones. That's why I use Automattic Kismet (Akismet for short) to capture them. But as far as enticing readers to post comments on your blogs, besides the simple fact that your blog is "good enough" for people to want to comment without any types of Enticements, what do you offer? There is a WordPress plugin called "DoFollow" - which removes the Code: rel="nofollow" from your comments. Would this be something you would consider as an offering to your readers?