Here are my characters and their roles:
Priest - Shadow main, Holy off.
Hunter - Pure DPS
Druid1 - Bear/kitty
Mage - Pure DPS
Warlock - Pure DPS
Warrior - Prot
Those are the classes that I'm playing mainly, right now. In the oder of main, main alt, second alt, so on so forth. Yeah... I enjoy healing. Yeah, I enjoy tanking. My favorite class? I am REEEEALLY enjoying my fire mage. Like, a lot. Like more than my priest (*Covers Rill's eyes so she doesn't take away priest captain-ness*)
So because I choose to play a class I'm really enjoying and having fun playing, I am just SOL? Not everyone is like me. My husband HATES tanking. He doesn't mind healing, but he prefers dps, because it's more fun to him. I have a friend who only plays tanks because he absolutely LOVES tanking. Another friend only plays healers because she loves healing. in four people, myself included, I have found that one prefers a pure DPS class/role to tanking/healing, and only one prefers either healing or tanking. I am the odd one out that doesn't mind doing any of them.
So looking at that, I then look at the behind-the-scenes stuff.
In guild:
DPS logs on. DPS asks - "Anyone want to run a heroic?" crickets... except maybe some other DPS... POSSIBLY a tank or healer.
Tank logs on. DPS asks - "OMG! TANK THIS HEROIC FOR ME!!!!!!1234" Tank logs off, rolls an alt, and hides.
Healer logs on. DPS asks - "OMG! HEAL THIS HEROIC FOR ME!!!!!!1234" Healer logs off, rolls an alt, sends a realid msg to the tank, the two level together and hide from the guild.
In whisper:
Mage/Warlock/Rogue/Hunter logs on and goes about their day.
DK/Warrior logs on, someone they've never met whispers "Are you a tank?"
Paladin/Druid logs on, someone they've never met whispers "Are you a tank or healer?"
Priest/Shaman logs on, someone they've never met whispers "Are you a healer?"
So then, the TL

R? Most tanks and healers are hiding from you because you DPS won't leave them alone. They log on and it does become work, because they feel OBLIGATED to run instances the whole time. Especially if you're a guild member and you're asking over guild chat, the tank or healer are then put in the spotlight. If they say no, they're rude and not helping the guild. If they say yes, they're roped into running instances when all they did was come on to do their dailies and they have a screaming child next to them that is sick and needs a diaper changed.
Ask a general question "Anyone want to run heroics? Need a tank, healer, couple dps." That way you're leaving it open for others to choose for themselves if they want to run an instance. That tank with the sick kid can afk and not have to worry about a group waiting on them.
Hope this touches all sides of the debate so that each side can get a better understanding of what the DPS is experiencing, as well as what the tanks/healers are experiencing.
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Q: Does Heroism make the same sound as Bloodlust when it goes?
A: Nope. Heroism has this sound like a gong being hit, and then something
like a triumphant cheer. Bloodlust makes a sound like bong being hit, and
then a murloc being strangled to death.