The fundamental distinction is between those who seek to encourage and stimulate progress (the left) and those who seek to slow it down or reverse it (the right). I know the notion of progress is not very popular among the left-wing crowd in this forum, but I don't see what other notion you could base the left/right divide on. All the others that have been proposed (equality/hierarchy, liberty/solidarity, individual/collective, etc...) aren't universal but related to a specific political-social context.
On the whole, this though there are certain situations where a "liberal" might be put in a reactionary position and vice-versa depending on how you look at it. (I would cite the 1896 election as an example where at the same time the "left" sought to "progress" away from the stratified capitalism, and at the same time "regress" back to a much more agrarian economy. Meanwhile the "right" sought to "conserve" the stratified capitalism and at the same time defend the "progress" of industrialization against reactionary agrarianism.)
