What are the two main differences between extrasolar planetary systems discovered to date and our Solar System?
The biggest single difference is the small number of planets in extra-solar systems. The second is that very few of them are in a life-supporting zone.
The interest in planetary systems is about the possibilities of colonization, or even the development of other life forms beyond what we know of on earth.
HAVING planets is a first step, and so far the systems detected do not have very many. The second step is for a planet to be in an energy zone that can support life (as we know it). So far, Earth is the only one!