You can confer some of your site's reputation to another site when your site links to it. Sometimes users can take advantage of this by adding links to their own site in your comment sections or message boards. Or sometimes you might mention a site in a negative way and don't want to confer any of your reputation upon it. For example, imagine that you're writing a blog post on the topic of comment spamming and you want to call out a site that recently comment spammed your blog. You want to warn others of the site, so you include the link to it in your content; however, you certainly don't want to give the site some of your reputation from your link. This would be a good time to use nofollow.
Thanks for bringing up this point - I agree Eric - competitive positioning can help you determine value that you bring to the table that your competitors dont.  I'm all for it.  Neilsen does some reports that provide awareness, likelihood to recommend, sentiment and other insightsfor your site/brand and your competitors. You can also pull some of that type of insight out of social listening platforms like NetBase, SM2, Radian6, Dow Jones, Nielsen, and so many others.  I've even done some hacked compeitove sentiment comprisons before using Search: searching for [brand or feature] + "like", "love", hate", "wish" etc. 

An SEO technique is considered white hat if it conforms to the search engines' guidelines and involves no deception. As the search engine guidelines[18][19][52] are not written as a series of rules or commandments, this is an important distinction to note. White hat SEO is not just about following guidelines but is about ensuring that the content a search engine indexes and subsequently ranks is the same content a user will see. White hat advice is generally summed up as creating content for users, not for search engines, and then making that content easily accessible to the online "spider" algorithms, rather than attempting to trick the algorithm from its intended purpose. White hat SEO is in many ways similar to web development that promotes accessibility,[53] although the two are not identical.

If you havent see it already, check out the links in shor's comment below - there are some great resources in there. In some cases you can also consider surveying your current audience or customers through email, on-site surveys or SurveyMonkey.  Be sure to ask for some profiling information that you can use for determining specific persona needs like age, sex, location, etc. (Probably best not to make it sound like a creepy text chat like I just did though...)  :)

In addition, you can also use tools like Majestic SEO to see who is linking to your competitors. Once you identify the links to your competitor’s sites, you can analyze these links, learn how they got them and implement a similar strategy for your website. For example, did they donate to a charity causing the charity to link to their site? You can do the same thing.
Thanks Brian for your article. I am in the healthy living niche. I want to team up with bloggers in my own niche where we can share material it makes sense to me. But I have my own unique message and that is what I have been devoted to! Dah! I see now that my focus should be on what is popular among my peers and add to this. I think I’m finally getting the picture! I am specifically into FOOD MEDICINE perhaps I should start writting about the dangers of a Gluten free diet! Not for everyone!
I completely agree that defintion of a target audience isa great first step, but would ask if adding in competitors to the analysis (mentioned here as a later step) helps draw out who your target audience would be via comparisons, i.e. showing who you are an who you are not - would be very interested to hear opinions on how this tactic can be used within the overall step in coordination with targeted keyword discovery.