First, something that will generate a player base is most important. From experience this has been competition, such as combat, spleefing anything that gives players a goal and the ability to become and brag about being "better" then other players. Although this does bring in the annoyance of complaints and cheaters it will benefit you in the long run as players are more prone to joining these types of servers frequently.
Secondly, originality. This isn't actually required, because it's a lot more challenging and more prone to failing; but take Shotbow's Minez or MCSG; extremely successful. There is a problem with this though, like I said it's a lot harder to manage as generally it'll require non-public resources such as a clean-cut website, a custom plugin and a unique idea. It's fine to have a normal pvp server, a faction server or heck another gamemode server with modes such as the survival games but it will not be as successful (unless you've done the correct advertisement).
Lastly, money. Unless you have a high paying job and can afford the server host being upgraded and paying for website fees and plugin developers etc you'll need a source of income coming from the server which is being worked hard on. Never, for example; give player's who donate godmode, maxed enchanted weapons and tons of diamonds per spawn, especially if your server will have a valuable economy. Keep it something simple - chat colour, server priority, voting weight (if you have some similar system) and/or really tiny benifets that don't change gameplay for other players putting them too much into a disadvantage.
