One think that makes me stay on a server is regular events and awesome members/admins/owner. I'd recommend having an event 2-3 times a month, depending on staff. I'd also say you should add a whitelist to your server, so it can try weave out the bad, immature players and help make your community even better!
One thing that makes me leave is grief, inactive admins/staff, bad community e.t.c, just general bad stuff you can get on a server. I would take this as a grain of salt, as I've never 'technically' left a server. I say technically because I used to go server hunting in hope of finding a 'good' server, and commonly I would leave because the server isn't cracked up as what the people say it is.
I would join your server, as you seem like a pretty cool guy, but I'm already on a excellent server, I'm not going to mention any other details to stop taking players from you. However, I'm currently busy on another server.
Apart from that, I can just wish you the best of luck with your server and hope it thrives.
~Arcapse