Mmm, love reading your blog posts, makes me want to get back into RP as well.
Headcanon...yes, I guess I have a lot of it really.
Druids - Mainly I see shape shifting as something slow and quite painful, not a BOOM, I'M A CAT! type of thing. I also visualise it as something that would really mess with you, you'd probably start taking on the qualities of a bear or cat, because you have turned into one. If I ever RP my baby bear tank, he will essentially turn INTO a bear. That means no talking, no human behaviour, and a constant battle within himself to come to terms with the fact that he is slowly turning into something else. Also, coming back to his usual form is something I would imagine he'd not willingly do, and that probably gets harder and harder as he stays longer in his bear form. An eternal battle within you where your feral, animal nature battles with your humanoid nature. I don't personally play either of my main druids as shape shifters, the healer doesn't ever go into tree form (except in raids when I have to to heal, but that's outside of my RP life) and my boomkin hasn't been a chicken since the glyph came out.
Warlocks - These are people who by the definitions of our times have sold their souls to the devil. They consort with the worst powers, they have twisted themselves into something that cannot be redeemed. They deal with demons who they have wrested from the Nether to do their will. I do not see most demons as being very willing to do your bidding (unless it coincides with their own plans), but they must because you have bound them with ancient words and spells, but most will always be looking for a way to release themselves, or trick you or to hurt you. They must resent the power you have over them, and a warlock must always be one step ahead to avoid any harm that a legion of angry demons would cause them. Secondly, warlocks are not and should not be accepted in any society, barring maybe the Forsaken. If you walk around with a demon out or in demon form or happily stating that you are a warlock, you are considered BAD, nobody normal wants to consort with you. You have shown yourself to be a deceiver, a trickster, a power-mad magician who gave up their souls to follow a path of power at any cost to themselves and others. You are effectively an outcast. When my Affliction warlock's mate found out that she was a warlock, he left her instantly, packed away their daughters and cast her away from his heart and their home, even though she had become one unwittingly due to wanting to repay a hurt he had been caused.
Mage - If you use fire spells, you burn your hands. You don't shoot bolts of fire or frost out of your hands at no personal cost. This also goes for Elemental shammies who cast lava bolts and lightning at their enemies, but unlike shammies, mages cannot heal their hurts. My fire mages have always had hideous, blistered and burned hands, although as they have been trolls it regenerates and heals fast, but that doesn't stop the pain when using the magic.
Priests - I agree with you, Aela, about Shadowform and have always visualised it that way myself. Also, Forsaken using Holy spells causes them intense agony. These are the very few who have the presence of mind and will to be able to channel holy light through them at a personal cost to themselves. The holy despises anything undead and here is an undead channeling it, it shouldn't make sense. It doesn't, but to make it less of a lorelol, I simply imagine that those are the very few undead who try against all odds to fight the evil in themselves, and probably lessen their "lives" by doing so.
DK - Similar to warlocks, yet perhaps even more horrifying. These are dead people who were resurrected for the sole purpose of bringing pain and suffering to others. Their old master is gone, so I don't really understand how they can still remain animated because his will no longer moves them. One of my DKs I roleplay as being a mindless drone. She is a corpse after all. When the LK was alive she did his bidding fully, without question. When he died, her mind became "free" to be used by someone else, and she is now controlled by her sisters who try to make her act like a living, breathing person. It's not really working, but as she is an animated corpse with no soul or mind or will of her own, this is the way it will be until her body decays. My other DK is the other end of the spectrum, I am trying to RP her with an echo of a personality that has returned now that she has her mind back. Yet she is still a dead being. All her relatives and friends shun her, she is an abomination. She hates herself, she wants it all to end. She is in constant torment because she is not fulfilling her purpose as a killing machine. She certainly doesn't hang around in pubs having a drink.
There are some other finer points and opinions I have, but these are the ones I most strongly feel about when I RP my own characters. And if I see someone acting in a way opposite to this, my character will react to it accordingly. I'll always be impressed when a mage doesn't scorch their skin when they cast a pyroblast, or marvel at the cat druid that popped in an out of their form as easily as if they were changing their socks. I will always shun warlocks when playing non-dark characters, and I will not consort with DKs unless forced to.