A lot of people use nofollow to cut down on Spam and avoid linking out to bad neighbourhoods. While this is a good idea for big and trusted sites it's a terrible idea for average joe's forum or blog. The correct way to run a startup blog or forum is to allow comments and signatures but to moderate them and to have warnings for spammers. Quality reviewing outbound links will improve your link profile as recently stated by google. Outbound links to quality and relevant sites help your search engine rankings. Having quality comments that are dofollow is better than having no comments or poor comments that are nofollow. Not to mention you will have return visitors who may provide useful information on your site (and click on your ads).