As a casual player, I normally don't stay on servers for long. Often, I'm "hired" to create some sort of creative structure or area with the help of my girlfriend and we do it because we want other people to be able to enjoy our work (when I'm taken on for creative work I don't actually play, I'm an op flying around building stuff). When I play, it's usually with a small group of people who I become friends with but only then will I come back and play with them but, even then, only in sporadic bursts. More often than not, I play MC when I just want a simple break from other games and/or when I want to build something creative with my girl.
Although I'm probably not your 'target market/audience' for feedback on a casual user experience, in such a circumstance, the most important thing to convince me to play with a group of people more than once would be community and creativity. If there aren't any projects my new friends want to plan out or work on, I get bored and go find some (sometimes on the server, sometimes elsewhere). In short, if we all have a goal and we all enjoy each others' company, we'll play together again. I met a group of friends in a random FTB game and we still talk but I just haven't played with them in about 1-2 months (I feel kind of bad because I laid out a cool new HQ style for them but we never finished the roof and final touches).