- Difficult to get starting gear - For most people, it's hard to get started as a tank. Gearing is an issue, because some tanks (DKs, warriors and paladins) need specific tanking gear, while even the leather tanks still generally use different stats to some degree, different enchants, different weapons for tanking than DPS or (especially) healing. This is a problem the gearing changes in Warlords should really help with.
- Where can you learn it? - Tanking requires a different skill set from DPS or healing. While proving grounds exist, they don't really teach the most important part of being a tank - reacting to other players. It can be hard as a new tank to walk into a dungeon having never done it before. That leads into the third difficulty of picking up tanking.
- Dungeons don't provide any sort of experience right now - With the wildly disparate gear levels on people running random dungeons, you can have a tank in 450 gear trying to hold aggro off of players in 580 gear. While it can be nice to be the tank in 580 gear, even you might have trouble when groups don't cooperate, run ahead of you, pull mobs half way across the zone, and generally simply refuse to act like any kind of groups at all. This is something I'm hoping the gear squish and ten levels will do away with - we'll all basically be on the same page when Warlords dungeons are being run.
The biggest problem, I believe, is caused by the rampant gear inflation of Mists of Pandaria and it's a genie that can only be put back in the bottle by leveling. Frankly, DPS players can ignore the damage in most heroic dungeons - the mobs don't need to be tanked. The average player in LFR gear is strong enough to kill trash mobs in heroic dungeons before they can kill him or her, especially if the healer is decently geared and willing to heal them when they get into trouble. This creates a feedback loop - players ignore whether or not a mob is tanked properly or not, pulling as they see fit, which can often spiral out of control. As gear levels increase, it becomes even easier for players to simply ignore the tanks. But since each random dungeon group is created, well, randomly, no two groups can be sure what the gear level of the various players is. This creates literally dozens upon dozens of ways a group can wipe, ways that a beginner tank simply can't react to or cope with.
A group with everyone in raid quality gear would be one thing - the tank might not be able to keep aggro off such a group due to the nature of the beast (since everyone knows they can burn down mobs before they'll take any damage, they'll run ahead and pull more and more) but at least said tank probably has pretty solid damage him or herself and can get groups once they're in range. But a group with an undergeared healer and DPS that try and imitate the player in 560 gear despite being in 450 gear themselves will quickly disintegrate, and when it does, the guy who just decided to give tanking a shot on his monk or the woman playing her druid again for the first time in months is left holding the bag, blamed for a situation beyond their control. The rapid inflation of gear caused by multiple raiding difficulties over multiple raiding tiers has left heroic dungeons in a position of total inadequacy, and the lack of new dungeons this expansion has left five player content with no scaling challenges - nothing gets harder in a manner that keeps tanking relevant. If all players can survive the damage put out in a dungeon, there's no incentive to let the tank take the hits instead.
As it stands, this isn't a fixable problem until Warlords come out, but I really hope it's a focus of the development team. Dungeons become outmoded as gear increases and fear of failure is reduced, because players who don't fear death for bad play lose a key incentive for good play. Tanks can't tank if the group won't let them, and a beginning tank can't learn what good play in that role is if their group isn't playing as a group. This isn't meant to imply that all the bad behavior is on DPS players, many a tank has behaved awfully in a five player heroic. It's simply discussing how the behavior creates a barrier for new and aspiring tanks to overcome.
Hopefully, in Warlords we'll have new five player content throughout the expansion. This does a lot to keep gear inflation in check, since the five player content can offer (and require) better gear to clear it and keep all players (including tanks) honest to a degree. No more "I'm going to pull everything from here to the first boss" antics which teach no one anything.
Gearing up a tank set is pretty easy right now, easier than it has been - you can effectively get a set of 496 tank gear in a day or two of Timeless Isle farming, which is more than enough to start in on heroic fives. The only real limitation is a weapon and for some tanks a shield, which you'll have to buy via timeless coins - they were upgraded to ilevel 489 in a recent patch, they're certainly serviceable for a tank in heroic dungeons. One hand and offhand pieces cost 10,000 timeless coins, while 2h weapons cost 20,000 coins. Hopefully in Warlords the need for offset pieces will be much reduced (not gone, since certain pieces like rings and so on will have bonus armor for tanks) and catch up mechanisms won't be as necessary. I'm hopeful on this score.
We know that proving grounds silver will be required to queue for heroics in Warlords, and it's my hope that a real effort will be made to get them into shape as a teaching tool for new and returning tanks to bone up on the mechanics they'll be expected to understand. I was very disappointed with proving grounds as a teaching tool in Mists - I'm hoping that the promised redesign will make them more useful in that role. We definitely need more places aspirant tanks can pick up the basics without dealing with complete strangers and expecting those strangers to be patient with them.
I'm hopeful that Warlords of Draenor will reset the importance of five player content and tanking in said content. I'm of the opinion that a strong five player content system is good for the game, and it's still my favorite way to learn how to tank.