Continuing my economic analysis of tanking in WoW (and it probably applies just as fully to healing).
A barrier to entry in economics are factors that keep you from entering a market where otherwise you could make a profit. They are the one time costs you a player has to pay (or overcome) to start tanking but that someone already tanking doesn't even notice.
Considering that MMOs have always had a shortage of tanks (and healers) its surprising that WoW's design has such high barriers to entry for tanks--higher than even EQ. The barriers are: class, spec, gear, knowledge, skill and psychology.
Lets say you're 80, on your first WoW character, and finally decide you want to tank. Chances are that you stuck right there. In WoW 6 classes can't tank (mage, rogue, warlock, hunter, shaman, priest) and 4 who can (warrior, paladin, death knight and druid). If you want to tank you have to give up your main character and start a new one. To add insult to injury, very little of what you have earned on your main will be useful to starting a new character (though more gets carried over now than before).
So lets say that our random 80 is one of the 4 right classes to tank--the next question is do they have a tanking spec? Do they have a talent spec that they can get to, to tank? Has this character gone back to the old world to train/retrain? Or have they spend the 1,000 gold to get their dual spec? In the log run, dual speccing is the better choice--so this barrier would be 1,000 gold and a trip back to the old world.
Ok now we have our random 80 with the right class and spec--but they still aren't able to tank: you need the right gear to tank: instead of damage gear, our player needs to have spent time and money to have a set of tanking gear. Even with defense out of the picture in Cataclysm, tanking gear will look significantly different than DPS gear: more stam, more armor, dodge and parry and it will be gemmed and enchanted completely differently. Mind you the optimal behavior--until you decide you want to tank--is to sell off or disenchant all that tank gear.
Ok now that our tank wannabe has passed the class, spec and gear we're ready to roll, right? Wrong--the player behind this tank needs knowledge. Everything from what his class skills are, where those buttons are on his UI, and what the cool downs are like to knowledge that leaks over into other barriers: do you know what a good tanking spec is? Do you know where to find one? Do you have enough HP? Armor? expertise? Hit rating? Resistances? (And more I am sure I have missed.)
Having passes all those checks--our tank still isn't ready: we need to know if this tank has the skills: does he know where his taunt button(s) is? (G9 on my keyboard). Does he know how to use his class skills in tanking situations? Does he know about pulling? Patrols? Looking out for healer agro on agroed ranged mobs? What about the pulling possibilities his group brings with it? LOS pulling? Staying top agro on 5 or more mobs? That he even has to stay top agro on most of the mobs? Does he have the situational awareness to respond quickly to changing situations?
And the, having satisfied all those prior checks--you have to ask is the player willing to take on the awesome responsibility for taking the fun and time of 4 other players in his hands? Rarely, I am reminded of what it is like to tank for the first time and get a set of nerves when taking on a new instance or boss. I doubt that is comparable to what people think of tanking for the first time if they know the game already--and how fast you can get feedback.