I think that the fundamental similarity that the two "star- crossed lovers" share is that they both are "star- crossed."  Their basic link is that they wish to act with a spirit of independence that is never full realized.  They both share a feeling of connection to one another that is fundamentally repudiated by their social setting of Verona.  Both of them act in a manner that rejects this social order, seeking to realize what can be as opposed to what is.  It is here where I think that their most basic and fundamental similarity can be found.  They both use their freedom to ends that prove to be self- destructive.  Both of them feel that they are acting in the name of something higher, more transcendent, and something that cannot be fully realized in the setting in which both of them live.  To a great extent, another similarity that both share is the idea that the only way their freedom is fully recognized is to act apart from their social setting.  Romeo must end up acting outside of his friends, and Juliet outside of her familial duties and obligations.  To these ends, both of them share common traits and characteristics.