One of the 7 essential meta questions
Straight-forward but difficult and very important question. Let's work together on defining this new site especially considering that we have a closely related Fitness & Nutrition site on the network (as raised in another meta question)
A great place to start is the definition of the site on Area 51. Also have a look at the F&N's FAQ and especially the questions they exclude. Some, like "Rules of Sports" is a great fit for us and an illustration that we should be able to co-exist with F&N.
Let's get started in an answer and modify it based on the community input.