1) an organized Spawn: if once you joined a server and you are mutilated by colours, random chat rules spamed by console, lots of books, suspended text, maps, mobs, annoying XP "level up" noises... Goodbye server. It would be just like this: once you spawn in, you are in frount of a wall. There are three buttons. One will give to you the rules, other the server info, and the third anything the Owner thniks it is important to put there.
2) Different chat channels: eveyone loves their privacy, and one way to get it (except from the console) is local chat.
3) Arenas: I think that every bending server should have at least one arena. And this arena has to be fair to all players. Enough dirt so Earthbenders can play, some water lakes in order to waterbenders don't have to glitch out and create their own water (also, bottles), a few mountains would be nice to Airbenders and chi-blockers and LAVA SO I CAN EASIER BURN THE DAMN FIREBENDERS!! >
... no lava, please. The only lava the arena should have is the one Earthbenders create.
4) Survival and RP worlds: griefing allowed, but not recommended (what you do to someone else, they can also do it to you).
5) A bending quiz would be nice: you start by answering some questions (not questions like "Do you drink water?" BAM waterbender) and at the end it would tell you what Element your personality "fits" the most, but you could also choose another Element or chi-blocking.
6) Respecting staff *here comes the plot twist* that desearves respect: Staff and the members of the server should be just like the Elements, balanced. Don't give too much "goodies" to your staff, because not all people is professional. Make sure that the people you make OP aren't just there to play around. Staff NEEDS to follow the rules. Before staff members they were just players.
7) Balanced Elements: I don't like servers that have too many technique of an Element and the others don't. If you are getting new seven Fire moves, make sure the other Elements also get some buffs.