I agree with Shoxio on this one, the absolute MUST is ROLEPLAYING. To filter out the power gamers and kids who don't want to do anything but run around and kill everyone, this requires whitelisting. Another key is to have an ACTIVE STAFF. Doing this puts you a step above most other servers where the staff are ridiculous OP and never play, for fear of losing their OP gear. Having an active staff helps you meet the needs of all your players, which is HUGE. I cannot stress how important having a fun player base is, and you only get that by being active. This brings me to my next point, which is a small number of players (relative to the space in the world). As a serious roleplayer, the crowded spawn areas on most RP servers is an immediate turn-off and I usually quit instantly because of the chat spamming. My final point is the one that really sets your server above the rest, comprehensive lore on your world. I understand this can restrict some things, but it is worth it in the long run, because your lore is what directs your world building. Every place exists for a REASON, so let players who explore be rewarded with a comprehensive knowledge of the world.
Sorry for the novel, I just have a lot to say, as I am currently working on a server myself, but it may be falling through. So if you need a loremaster and builder, PM me.
Thanks for reading,
Freedom's Flame.