I don't think a forum has to be unique, personally. A forum is about the members who use it, more than any idea. Every forum has its own set of members. So in a sense, all forums are unique, and the best forums are determined by the quality of the people who use it. A forum doesn't have to be unique, it has to be fun! And that is why I use the forums I use.
You're not going to get very far in that case, because there's no incentive for a stranger to join. There's far too much out there to get by on simply "our community is great", because you can't effectively convey it and there's plenty that have much more to offer.
Uniqueness is also a required basis for
any advertising to be effective.
I disagree. If one's community is far better than other forums, and the members on that forum are much more involved in the discussions, they're going to recommend the forum to other people who will eventually join because they know someone there. How many forums have you joined, where you are an active member, where you knew absolutely no one? The members, the discussions, the qaulity of the posts, and the atmosphere is what makes a community unique and different. At this point, each community is probably using the same plugins and add-ons that hundreds of other forums use. It's the members that make the community different, and it's the networking that said members will do that will grow your community.