It comes as no shock to a longtime player that WoW's social culture is riddled with a number of real-life counterparts, and one of the more troublesome is something called the barrier to entry. In real life, this refers to the difficulty of becoming a qualified professional in a given field, and there are some jobs where the barrier to entry is very high indeed.
Take neurosurgery, for example. Ideally, you want to be completely sure of someone's aptitude for the job before you let them take a buzzsaw to your skull. Society relies on the grueling education and residency required to be a neurosurgeon to weed out anyone prone to use of the word, "Oops."
While there's nothing in WoW that comes close to the seriousness of getting a competent surgeon, most players would acknowledge that there are similarities between the RL and in-game version of the "barrier to entry." I'd argue that the comparison is strongest when you're discussing tanks. Tanks, and more importantly, beginner tanks who could potentially ease the tank shortage that causes lengthy queue times for DPS in the Dungeon Finder, have to hurdle a series of problems in the effort to become geared and experienced. Some of these problems are the result of deliberate design choices on Blizzard's part, but the larger share is the consequence of a playerbase that needs tanks but is (ironically) hostile to beginners.
Everyone wants an experienced professional. Nobody wants to be there for the learning process.
And if you're a beginner tank, there's a lot of crap in your way that Blizzard didn't put there.
What is the tank's "barrier to entry?"
The combination of gear and experience required to tank content without threat-capping DPS, straining the healer, or endangering the group's survivability.
While a similar barrier to entry exists for DPS in advanced content (e.g. DPS players unable to surpass 1.5K DPS are not likely to clear heroic Halls of Reflection), it's difficult to argue that the group/raid suffers more than it would with an undergeared and inexperienced tank. A fresh DPS at 80 can jump into heroics without seriously affecting a group's chance of success, or having any measurable impact on the run beyond the speed with which mob packs die.
Essentially, a DPS player's gear and experience largely affect the efficiency of a run, whereas a tank player's gear and experience affects its efficiency, ease, and odds of success.
Why is the barrier to entry a problem?
In some respects, it's not. Very few players want someone with a limited understanding of the role tanking any of the game's more difficult content, and a beginner tank who fails to appreciate the job's stat requirements is unlikely to advance beyond the 5-man level (and may very well wipe groups even there). These hurdles exist in part to distinguish committed players from those who do the job merely for the convenience it affords them in the Dungeon Finder, and that's good design.
However, the barrier to entry is problematic in that it limits the supply of available tanks. There are a number of skilled and experienced druids, warriors, death knights, and paladins who could conceivably tank but don't, usually for one or more of the following reasons:
Being immune to mob and boss critical hits is a big deal, and rightly so. Bears have an advantage with 3/3 Survival of the FIttest guaranteeing crit immunity, but plate tanks have to assemble a set of gear with specific stats. Being crit-immune is virtually mandatory even for the earliest heroics, and this typically requires an oft-expensive investment in crafted +defense gear. So before the tank even sets foot in a heroic, they will have to spend time and gold ensuring their stats meet the minimum acceptable threshold for survivability.
Threat is inextricably linked to the quality of the tank's gear, and it's also less straightforward than either damage or healing. If you get a 9K crit on a mob, you did 9K damage to them. Healing Wave hitting for 15K heals a player for that amount (or whatever it took to get them to full). By contrast, how much threat an ability produces on your target is not immediately obvious due to threat modifiers altering how much threat your abilities actually generate.
New tanks worry (and rightly so) about holding aggro against DPS while they're gearing up. The need to reach the +defense cap is an overriding early concern in a period where DPS will be piling up more spell power, attack power, hit, and crit.
There's an art and finesse to a good pull that requires a player to know a dungeon, mobs, and mob pathing intimately -- and it's easy to botch this even if you've seen the dungeon many times as a DPS or healer. You have to know what mobs are in each pull, the abilities at their disposal and the effect they have, where caster mobs can be coaxed to move if you can't silence them (a particular bear irritant), and the most opportune moments for a stun or silence.
In the age of AoE, you also need to know how to salvage a pull that goes badly for reasons that have nothing to do with you. DPS frequently opens up on mobs that aren't yet positioned, with the unhappy result of ranged mobs stopping in their tracks to shoot back, or melee enemies peeling off you in search of a tasty-looking clothie.
If you stand in fire, you shouldn't be tanking. When does the mob drop an AoE? Do you know where you can move the enemy without ranging your healer? Is your computer able to get through your character being in the middle of boss attacks and player spells for an extended period of time?
This one is tough to explain, but I'll try to gloss it; a good tank is often recognizable in what doesn't happen to the group as a result of the decisions they make. They've had sufficient experience to know which mob packs are frequently pulled by accident, and they save interrupts for the mob abilities that really count. They reroute the group (generally without overtly acknowledging this) to avoid trouble, look ahead to see if a pathing mob is likely to aggro in the middle of another group, and save their stuns and interrupts for the Ahn'kahar Spell Flinger charging up a Shadow Blast, an Anub'ar Skirmisher at the moment they drop aggro, and the Ghostly Priest beginning a Dark Mending.
A tank who anticipates player behavior (and can swiftly compensate for their own mistakes) is all about the wipes that won't happen, and the group will leave the dungeon none the wiser.
Out of all the barriers to entry, I would submit that the need to deal with players who are hostile to beginning tanks is the single most painful -- and unfortunately intractable -- issue. All of the aforementioned requirements are things that come with enough time and practice, but tanks have little control over others' reactions to the learning process.
Most healers don't find beginning tanks all that difficult to heal if both players are sensibly geared (even if the gear in question is mostly blues). But a tank outgeared by his/her DPS will find it virtually impossible to hold threat (particularly AoE threat) against them, even if the tank's rotation is absolutely perfect. Understandably, most players are reluctant to put themselves in situations where failure is preordained. They're equally reluctant (and who can blame them?) to shoulder the abuse that often occurs when this happens.
Cataclysm stat changes and the future of tank population
The upcoming changes to tank crit immunity (which should automatically occur after shifting to Frost Presence, Defensive Stance, etc.) are directly aimed at reducing the barrier to entry by removing one of the more onerous gearing requirements for plate tanks. While I was initially hopeful that a similar change to Survival of the Fittest (in patch 3.0.2) would result in more bear tanks, I was unsettled to see a dramatic decline in druid tanks when Wrath hit. Druids have it easier than any other tank when it comes to satisfying this first and most important requirement, and yet they remain among the less-played tanks. To remove the necessity to gear for crit immunity is apparently not enough to encourage the population growth of a tank spec.
Blizzard has worked hard to remove many of the issues that made tanking an unattractive job for players (to the point of designing a hero class with unprecedented flexibility concerning spec and talent choice), but I can't help but think that the larger share of responsibility for tank population ultimately lies with players.
Everyone wants more tanks to queue in the Dungeon Finder, but the ugly reaction that often greets a beginner actively works against the likelihood of DPS players getting groups more quickly. The game is meant to be fun, and it's not fun for a newbie tank to wade through a sea of impatient players in the effort to get gear that will allow them to be humiliated less often. Rather than the positive reinforcement of doing more damage or healing more efficiently, a new tank's real desire is to avoid being kicked over a low gear score or abused for failing to hold aggro against better-geared DPS. They can do this eventually with better gear -- or they can avoid it entirely by not queuing at all.
Be kind to your newbie tanks, my fellow players. At some point down the line, the time you save may be your own.