Firstly, I look at the staff and community. If the staff abuses their role too much I will just leave. Rules like "don't disrespect staff" I absolutely hate because 1. Staff are players too. If they're gonna raid someone that person deserves the right to call them out for it and be pissed. 2. Staff abuse this way too much. Any argument / disagreement you have with them they just throw the rule "Don't disrespect staff".
So yeah, if that rule exists I'm not playing on that server. It gives staff waaaaay too much power.
The community is important too. If the community is nice, helpful and you can have fun with them A+, but if your community is ignorant or aggressive to outsiders, generally all sticking together like one big clan to gang up on anyone new, chances are I won't play there.
Player count;
I don't like to join a server with anymore players above 40.
I don't feel like I can be apart of a community that big, there's just way too much noise and you can hardly ever be heard. The chat is spammy and it's hard to keep up with what's happening.
Balanced;
The bending config needs to be balanced. I don't like it if you can buy OP moves like blood bending, avatar state etc.
I also don't like it if the server punishes new players by having unarmed enabled (McMMO), it makes it a real grind and spoils the fun of bending fights.
Performance;
Not much of a description needed. If the server is constantly lagging, it effects gameplay and makes the experience bad.
Features;
Basic features like an arena is needed. If a server doesn't have an arena, I will leave cause that's my favourite thing to do on bending servers. Aside from that, added unique features to keep me interested wouldn't hurt.
Easy on the eyes;
If every single person has a multi coloured /nick it's quite annoying and hard to read.
Generally making the server look professional and clean makes the experience better.