That I follow, or wish others would follow:
1. If you're going to afk for more than a couple minutes in a dungeon, move first to within Rescue range of the lock-out area so you can rejoin the fight upon return -- though a rez may be necessary first.
2. Anyone can do anything, insofar as their kit allows. Know the short-term goals of each roles -- where to gather, when mitigation is necessary, etc. -- and the conventions to accomplish them. Play around the specialist, as they'll have point on the specialized task (healing or tanking), but know that if your toolkit allows, your responsibilities over even specialized tasks do not end just because you have a specialist for that task in group; only when there is an effective cap or the action comes at greater cost to some other function (though these will each be more often the case than not, especially in the case of a DPS healing, for instance) do you let a specialist handle the task completely.
3. Just because something is your specialty doesn't mean someone else can't do it better. A cross-Clemency during Requiescat after a combo-finisher will do a hell of a lot more than your Cure II, while allowing you to Holy spam mass-AoE. A mana-capped DA-Abyssal likewise will do more healing than any of your particular spells, but its ability to be spammed will only last as long as there are many mobs and either Blood effect. Be aware. Know others' toolkits.
4. Know your AoE and mobility ranges. While it's on the BLM to be within AM range if he wants to use you to escape, do not beg for healing after moving out of AoE heal range from its optimal position; unless forced by mechanics that you couldn't prepare for to reach an better result, that, too, is on you.
5. Always be doing something. Unless facing Blighted Bouquet or Pyretic. As long as you're not wasting uptime on restorative abilities, it is virtually impossible for casting nothing or using zero TP in a given GCD to be worthwhile (only exception -- mass-AoE when resource-starved, which usually comes down to too low of DPS and/or wasted uptime on restorative abilities).
6. If someone takes issue with your performance, your default behavior should be to reflect on your rotation, positioning, and engagement -- not to attempt to have them banned just because you made a normally easy pull impossible.
That I see:
1. Feel free to afk without warning.
2. Assume that if you are the tank or healer, then you are free to hold hostage the time of all others in your group. See rule 1, but know that it now affects all 4 players -- or so you would think.
3. Report any tank who says so much as "I need at least a couple of those 12 mobs to be dead within the 30 seconds that my CDs are up". Try your best to get him banned; he may have just slighted your willingness to learn or perform accordingly. Do not ask for advice. Equatable cases for non-tanks.
4. Pop sprint, run towards boss, and then wait until Sprint has faded. Bonus for popping CDs before afking.
5. If you converse, be sure to stop doing anything during that time, rather than using the ample up-to-2.5 seconds between GCDs to gradually continue your text.