You might have listened about a WordPress plugin called SEO Smart Links, it provides automatic SEO benefits for your WordPress blog in addition to custom keyword lists, nofollow and much more.
Urtak is a WordPress plugin that creates conversations powered by questions. It brings both simplicity and structure to any online conversation, it simply allows your users to ask questions to each other. It gives you chance to understand your audience better and know what actually they are thinking about your website, this way your site can engage more people.
Add a Twitter retweet button (similar to techcrunch) for your WordPress posts and archives that shows all tweets and enables retweeting. If you want you can also add adds tweets as comments and trackbacks.
Some people don’t use Gravatar, some don’t use Twitter and there are some who don’t use Facebook also and when they leave comment on your blog they leave a mystery man image (or your site’s default avatar) in their comment which sometimes does’t look nice. But you can allow them to choose their comment avatars manually by selecting a avatar from their social profile, email or choosing it from a list of your site’s custom avatars. I know about few plugins that allows you to do so.
Reward your WordPress site commenters by automatically adding a link to their latest post at the end of the comment they have posted. It Encourages a community and discover new posts.