I think all my favourite long term stories have one thing in common, they all deal with day to day stuff, such as school, work, hanging out with friends, sports, all those kinds of things we do every day of our lives. A story doesn't have to be full on drama all the time, it's actually nice to see the characters do the daily mundane stuff, I think there is a lot of mileage to be had with that. Relationships also develop over time, it's nice to see how the couples change over the course of a story. Obviously that can get boring over time, you need some drama now and then, but three that come to mind, New Kid In School, A New Life and Storm Front all seem to have that balance and that is why I think they have all continued as long as they have.