
Someone on the main site of SMF asked this same question and I answered this way:
Think about your site like a restaurant and ask yourself... What are you serving that will make a person leave a restaurant they may already know and frequent to come to yours? You have to win them over.
In other words, what you are offering has to be better / more enticing than another site in your niche. If your site is about apples do not expect someone interested in oranges to join, so focus on what will attract 'apple loving' customers.