I understand your concern and its a very valid one. Comments show activity on the blog. I know a few plugins that might help and I would be using them some on my blog too. Also the No-Follow tag issue can be balanced with the 1st plugin that I have listed.
1)
Link Love is a plugin that turns off nofollow for all commentators who have made at least 10 comments. Not only that the number of comments can be changed from the plug in options. This might actually solve your concern about too many outgoing links.
More info here :
http://www.allpassionmarketing.com/software.html#Link%20Love2)
Comment relish is a plugin that sends a thank-you email to your blog’s first time commentators. This encourages them to come back again and comment regularly. This is a really good idea. Again the message can be configured.
Can be found here:
http://www.justinshattuck.com/2007/03/19/comment-relish-wordpress-plugin3)
Subscribe to Comments is a plugin that allows commenters on your blog to check a box before commenting and get e-mail notification of further comments. This encourages readers to come back and take part in the conversation, if they get a follow up or an interesting comment.
It can be found here :
http://txfx.net/code/wordpress/subscribe-to-comments/ 4)
Show Top Commentators As the name indicates this Plugin encourages discussion from your readers by displaying their name (linked to their site without a nofollow tag if desired) and number of comments on the sidebar. The Plugin also gives you the option to reset the list every hour, day, week, month or year.
Can be found here :
http://www.pfadvice.com/wordpress-plugins/show-top-commentators/I am sure with the use of any of these plugins you would see an increase in number of commets on the blog.
I am going to experiment with them on my blog as well.
