It isn't just content that can be unique, though unique content is best, but if your site is based around public domain content or something similar there are steps you can take to make your site more unique. You can change the user experience of your site by presenting the content in a unique way, or you can offer custom functions for the manipulation and display of that content.
By far the easiest way to make public domain or datafeed content unique is to add user submitted content to it. The most common method of doing this is through user reviews. By asking users to submit reviews on your content or datafeed products you will create a database of unique content that adds long term value to your site. Even if a competitor creates a site using the same sources you did they will not have your hundreds or thousands of user reviews and as such will have to play a lot of catch-up. Even while they're playing catch-up you're adding even more reviews so the chance of them catching you is slim, and even if they do catch you you still get plenty of traffic at #2.
The other thing you can do is integrate a forum with your site. A forum is the holy grail of unique content. Even if someone had a forum on the same topic with the same amount of activity it cannot destroy your forum because another forum can never duplicate the community that your forum has. Forums are held together through user interactions, and while other sites can also offer interactions, they will not be with the same users. As such the chance of someone leaving your forum for another on the same topic is very slim. They might perhaps visit both forums, but as long as their friends are at your forum they will stay.
A forum, while not the most profitable type of site, has more potential for long term survival than any other type of content website, and adding a forum to your site is the best way to ensure long term survive, with or without unique content on the rest of the site. The other thing that could really kill a forum is obsolescence. If the industry your forum is about suddenly changes faster than your forum can adapt it could go under. If your forum is about a passing fad it could go under as well. Finally, if you alienate your established members to a large degree your forum may fail as well (though if you keep it up it should bounce back in time).
Just because your site is built around commonly found content does not mean it cannot be unique. By mining your visitors for content you will eventually create enough uniqueness to allow for long term survival of your site.