Invisibility scrolls: Many quests can be run "discreet". Not only is this very fast, it also gives you a "discreet" xp bonus. XP per min is better than xp per quest.
UMD: cha of persuasion helps. Persuasion can be crafted ml0. +4 cha ml 5. Bunny hat is a cosmetic hat that also gives +3 UMD on scrolls (but does not stack with persuasion).
Cure serious pots: Heal better than any pug divine (Much better than low level hires even...), especially a fvs. If you are playing with a guildie divine, he probably wont heal you either and then proceed to call you a noob when you die. Until about level 12 this is true. Sometimes this is true until level 18. FYI i do not play a divine.
Remove Curse/Disease/Poison/Blindness/Restoration pots: Should maintain at least 100 pots of each. If a divine cant be bothered to heal you, then why do you think that they will cast remove curse on you ?
Heal amp: you want as much as you can reasonably get.
Silver flame: Grind out the favor when it gives xp. Get silver flame pots. Be self sufficient. A friends quote "A good melee can self heal, a great melee is self sufficient".
Deathward: If you are playing a class that has a specific buff, say.. Deathward then it is good form to pass that buff out. If you can cast Deathward and you die to finger then you should be embarrassed and possibly delete your character. I see this happen a lot. Unlike some games, one does not need to stand still to cast spells so start running and pass buffs as you jump - space + buff spell. If casting masses, and people run off, then it is their problem.
GH: Contrary to popular believe, people do not ask for GH for the +4 to attack, generally it is to prevent Fear effects. Fear stops people using the aforementioned pots to self heal. If you are a bard, then you should definitely be passing out GH. This is probably more or less the only reason they let you in the group anyway (applies to 85% of all bards). Most people make sure they have a gh clickie or can UMD it.
Cleric/FVS: Carrying Harm can keep those pale masters alive. Keeping them alive is a good thing, especially if they are ccing. Agreed that many are arrogant SOB's but keeping them alive (and thus yourself) is a great way to gloat. The intelligent ones will thank you for it. The stupid ones will abuse you. Clearly come back to "if you knew what you were doing then i wouldn't have to Harm you...". Harm also clears stat damage on a pale master. Especially good for those IQ quests, Von2, Von5, etc. There is a ring that drops in Accursed Ascension that can be unlocked to give 5 charges of Harm.
Druid: Mass regenerate is possibly my favourite spell in the game. Throw it all the time!
If you are asked to pike, buff everyone as best you can and then pull up youtube or the forums for some giggles. Don't run in and die.
If someone is incap, do not stand on top of them and fight.
If you happen to die, stand next to your soul stone and not on it. Do not run away from your stone unless someone who can res has asked you to.
If you join a Zerg LFM, don't stay in the quest and loot all the chests when 5 other people are standing at the door waiting to come in again.
Litany of the dead: Use invisibility to reduce the number of mobs that must be killed. This gives extra xp.
Rainbow in the dark: In the caves, run to the end and kill everything. This saves on SP. If you die jumping the pillars, get to the other side and then take your res.
Any escort quest: Bring a divine hireling.
Abbot: If you do not know what your role is, or how to do it then say so. There are no stupid questions, only stupid people who fail to ask what the hell they are meant to do. Abbot in particular is a raid where the tolerance for failure on "easy" roles is very low, possibly getting you banned from future Abbots. However stating that you are willing to learn will more often than not be met with careful explanations and possibly even being edumacated.