Not having goals. As someone who's very ambitious and likes being active in my community and environment, I don't want to be with someone who has no goals. Or doesn't at least think about the future. That's hard to come by with guys and girls my age. And if they don't have plans or goals, be honest about it and don't try to just go with the flow about things like having kids, career, or getting married. Just generally being immature is an ick I guess.