I frequently brine chicken and pork, and mostly it's just salt and water, sometime sugar, and occasionally garlic.
But I've seen all sorts of other "goodies" people add in, like spices, herbs, sometimes oils, alcohol, acids, fruits, vegetables, etc.
The question is, what actually makes it into the meat? Salt and sugar seem to make sense, and acid or alcohol may affect texture, but I have difficulty imagining that the other brine ingredients are adding any more flavoring than on the outer surface of the meat.