I'll argue that Situational Awareness is the most important skill you can ever master in WoW, or even in most situations in real life. That means, having a complete overview and understanding of the situation, and know what actions you should perform to best solve it. Epic skill, no doubt.
Given you define a hybrid as a cross between two or more roles, all classes in WoW are hybrids. Well, that depends on how you define the roles. I define the roles as
* Crowd-control (CC): Making sure you're in control of the situation and no the opposite way around.
* Tank: Really just a specialized form of multiple-mob CC.
* Dps: Anything that helps the enemy health bar move to zero faster.
* Healer: Anything that helps your ally's health bar move to full faster.
That makes all classes in WoW hybrids. Even rogues. They can decide to spend their opener and combo points on stunning (CC) or damage. Now, all hybrids (and thus all players) must decide how much hybrid they want to play. There are two traps of a hybrid: Pidgeonholing yourself, and overreaching yourself.
Push yourself into only one designated role, and you've pidgeonholed yourself. In many cases, that's fine. A designated healer doing nothing but throwing around healing spells or a designated dps only dealing damage will usually work fine. But when all hell breaks loose, she/he's going to be suboptimal. More so in 5-man or PVP than raids, though.
A classic example is a healer (not the role, but a player designated to this role). Despite having the responsibility of expanding the green bars of any friends, they also have damaging spells on their toolbar.
Take a boss like the Headless Horseman. During most phases, the dps must take his health in full body form to zero. But wait, that doesn't really kill him. Then his head falls off, and returns to his body in about 5 seconds. When it returns to his body, he is at full health again. If the head doesn't take damage during those cruicial five seconds each time his body "dies", the circle will go on forever. At least until the healer is out of mana.
Now, during the phase where the head is vulnerable, noone's taking much damage (in fact, noone's hardly taking damage during the fight at all except when he launches a burning pumpkin on someone's head, but that's beside the point.) Given that the healer has mana left, he should burn damage on the head. Even if it's not a lot, everything helps getting the boss down faster.
A better example is PVP. Say you're in AB, and 2vs2 fighting over a flag. Suddenly, one of the enemies are really low at only 500 health left, and the other one is about to heal him. If you (the healer on your team) limit yourself to only watching the health bars, the other healer will succeed and you'll still be fighting 2vs2 and perhaps loosing. If you, on the other hand, throw a Shadow Word: Death on him, you might suddenly find yourself in a 2vs1 which increases your chances to win a lot.
Perhaps the best example is a feral cat druid dps in an instace. Optimally, he'll pop into caster and throw out some heals while the healer is CC'ed or OOM, and he'll pop bear and catch a loose mob if nescessary. If he doesn't, he's playing suboptimally. He's pidgeonholing himself.
The other trap of a hybrid is overreaching yourself. If a druid tries to heal and tank at the same time, he'll (usually) overreach himself, leading to his death and neither healing nor tanking.
It's not as uncommon as it sounds! Take arena 2vs2, two dps+healer teams facing each other. A warrior on the first team wants to kill the other dps, but can't loose his own healer. Thus, he'll have to balance between spending time and resources dps'ing the enemy healer, and intervening/intercepting/hamstringing (CCing) the other dps to buy his own healer some precious time. Of course, all the while surviving himself.
In an instance, a rogue trying to CC and dps at the same time might end up failing at the dps, thus not getting the mobs faster down that the healer's mana bar.
It's also possible to overreach a single role, or even others. A healer trying to keep up the warlock pulling aggro and the tank at the same time might end up loosing both. A tank that pulls too many at the same time is overreaching his or her healer.
The best counter to both of these traps is... Situational Awareness. Always having awareness of what's happening around you. Understanding the consequences of your choices (or lack thereof). Know when you should switch your role, and when you need to stick to it.
I'll share a few examples of how being flexible can save the situation:
- Any dps in a group can attempt to "tank" a mob or even several to save the group from wiping. Most likely, he or she will most likely die in the process, but the group might still be able to pull it through. Given the choice, what is better between the entire group wiping and just you dying?
- Most dps classes can choose to CC more and dps less.
- A tank can decide to use his emergency buttons immidiately rather than waiting to get low on health, when she/he notices that her/his healer is CCed or low on mana. (Okay, that's not really being a hybrid, just having awareness.)
- A healer can choose to let someone die in order to concentrate on someone else, rather than loosing both.
- Any dps can bandage.
- ... but so can a healer, when he's out of mana.
- A tank that has his threat well established can spend more resources on dps rather than threatgenerating.
- A healer can pump spare mana into damage.
- A healer can often CC. If you think you're on your way of wiping, what's the harm in trying to psychic scream the mobs away to give you some breathing space? Sure, they might pull more, but weren't you about to wipe in the first place? Isn't it better to do something that might work rather than not do something and garantee a failure?
- The same applies to tanks, at least warriors.
Bottom line, Situational Awareness is a skill to keep practicing until the end of your career, or even your life.
I'd also like to recomment Out of Mana's Confidence and Humility: part 1 and part 2.