You may have heard that running on grass or trails is much better for your joints and muscles than running on paved surfaces, and this article confirms that running off pavement is the much better option. There are so many trails to run on with different components to them all, whether that be hills or many obstacles, allowing you to evade getting bored by being able to run a different route every time. Not only is this variety good for your training for your body, but also for your mind. You have to map out exactly where you're going to step next in order to avoid obstacles, which in turn works out your mental fitness as well. As a trail runner, you'll learn what needs to be done in order to be successful, such as being quick on your feet or how to overcome a steep decline.
I found this article easy to relate to as I enjoy running myself. I also participate in Cross-Country running so I know what it's like to run on trails. Even though I am familiar with the subject of the article, I didn't find it particularly useful as it didn't really provide me with anything I didn't already know. If I were, however, someone who runs on pavement, this article probably would have changed my views and caused me to start running on trails.
Ottawa Outdoors Magazine (summer/fall 2009)