There was an interesting article recently that suggested that most habitable-zone exoplanets could be considerably more massive than Earth, making them much harder for a potential alien civilization to leave. Their gravity would be higher, and that has a magnified affect on rocketry because a rocket has to carry its fuel with it.

I'm not sure that totally holds water - after all there are theoretical ways into orbit besides rockets - but it seemed like something worth thinking about on this question.