I'll share some of my unwritten rules for FFXIV, not just from playing FFXIV, but from other mmos. Mostly pertains to me playing as dps, rather than healer.

1. Try not to steal aggro (unless the tank is actually ok with you pulling and bringing mobs to them, and your team mates are happy with this as well, it can be helpful sometimes). Either when I'm playing dps and/or healer, I will follow the tanks lead in what mobs to attack (and to watch my aggro) I assign a hotkey to "Assist Target" for this.
2. If you got utility skills that will help your teammates, please use them! On SAM I'll use Goad for whoever needs TP, for example.
3. Stick close to the healer (unless you need to split for mechanics etc), just in case you need to stack for heals, buffs etc.
4. Watch your teams speed and adjust accordingly, DON'T race on ahead, unless you wanna pull mobs and fold like a wet paper bag >.> Rule of thumb, use the tank's position as an invisible line you must never cross
5. Lastly (probably could think of more though to be honest lol) communicate with your team... I'd say that's the most important unwritten rule.