Depending on your personal preference, you may place the heaviest burden of a group on your tank or healer. While it's true that they carry a huge load, and I've even seen a single tank/healer combo kill Hiegen, the DPS has to be up to par as well.
Unfortunately it seems that the hardest two roles to fill are tanks and healers, and guilds are making tough choices about who to go with. Here's my pros/cons of each type of tank - and why I'd take one over the other. If you're just rolling a tank, think about your play style before picking.
The #1 tank of choice in Vanilla (most druids and paladins were relgated to healing duties), the warrior has come a long way in five years. This is your standard plate wearing, shield smashing, in your face tank. The warrior depends on plate, his shield, and a good balance of mitigation stats to keep him standing. The warrior took a hit in early BC due to the DPS output of feral tanks, and the ease of AoE farming from Paladins - but they were still the kings of raiding. This was especially true for Illidan where only the warrior could put up the necessary defences to avoid the shear - every time.
Pros: Best closer. Between Intervene, Charge, and Intercept - no other tank can close the gap as quickly as a warrior. Using Bezerker Rage they are also the best suited to tanking bosses that fear a lot. This was especially true when a mob would drop agro when you were feared. They have the only spell reflect of the tanks, and have access to a wide range of interrupts and stuns.
Cons: Little to no self healing/cleansing. Suscepptible to snares, and shouts only last a few minutes. Heroic Strike spam has been blamed for more than one case of carpal tunnel. Run the risk of rage starvation if they over gear an instance.
Foregoing a shield, the DK wades in with a two-hander - or dual wielding if they gear correctly. Using a mix of magic and disease, the DK can quickly establish AoE agro. Many DKs have gained a bad reputation as being unable to tank, simply because they thought they could just throw on any old gear and talent as they chose. DK tanks are plentiful, good ones are rare. Most of the great DK tanks were tank re-rolls who already knew the role.
Pros: Even with the nerfs, DKs have more absorbtion and mitigation tricks than a rogue with dirty deeds. DKs have ranged silence/interrupts, pets, and many group buffs. Unlike warriors, DKs can bring thier enemies to them - and there's nothing like seeing an Abomination fly across KT's room.
Cons: DKs are still fairly new (compared to the other tanks), and so are still being regularly tweaked and modified. With no shield they are heavily dependant on dodge/parry, and can quickly fall behind in their TPS if they make poor runic power/rune choices.
Druids forego any type of weapon, and instead parry with their faces (yes, I know they can't really parry). Stacking dodge and stamina, druids were the only tank in BC that couldn't avoid being crushed - they were expected to just eat the strike. Bears us a rage system similar to a warrior, and have many of the same debuffs as their rage sharing bretherin.
Pros: Mastering a druid tank is probably the easiest of all three tanks - at least from a gearing and playstyle (tank only) standpoint. They are defense capped by default (due to talents) so only need to focus on stamina and agility/dodge. The mini game of staying defense capped while juggling mitigation and hit is removed. Druids can also provide a lot of utility via buffs, battle rezzing (tough if you're the solo tank), and innervates.
Cons: No matter what you do, you'll never look different than you did at level 10 when you first get bear form. You'll spend all your time staring at the same shaggy butt, and occasionally switch to caster for some reason. Be prepared to be mocked by those that don't know any better, and be ready to defend your rolls against rogues.
Bearers of the Light (yes, even the Blood Elves), Paladins use a mixture of magical and physical damage to generate damage and threat. Fortunately after BC it was no longer necssary to use caster weapons to up your threat/dps, and Paladin's now share the same stat needs as warriors (and to some extent DKs, though we can use block).
Pros: High armor, stamina, and the ability to use shields gives Paladins excellent mitigation and survivability. Coupled with blessings, hands, and the ability to AoE taunt off another player without targeting the mob - Paladins excell in AoE fight situations. Paladins also have excellent snap agro through the use of Avenger's Shield and Shield of the Righteous. Being able to cleanse and use Hand of Freedom on themselves, Paladin tanks are rarely slowed or snared. Thanks to Sanctuary mana is rarely an issue. Right now Paladins have the most OP emergency talent with Ardent Defender - which is the equivalant of having a permenant in combat rez available - without actually dying. Bubble hearth - nuff said.
Cons: No closer of any kind, and limited interrupts. HoJ can be talented down to 30sec, thanks to Holy paladins speccing prot for PvP. With no stat stick (gun) available, it can be tougher to reach the necessary caps for defense and hit, but the bonuses from the libram can make up for this.
So which one is the best?
Like most things in WoW, this depends entirely on who's sitting behind the keyboard. While some tanks had advantages due to cool downs or other mitigators, every tank is capable of tanking every encounter in the game. Most guilds stuck with thier same tank going into Wrath, regardless of class. 25th November/Ensidia didn't ask Kungen to re-roll, and neither should any one else.
As long as the tank is uncrittable with 540 defense (400 and the talents for druids), and has 262 hit rating (slightly less for Draeni), your tank should be ready to go at the appropriate level of content. In my opinion anyone who picks a tank based on class (raid leader/GM) either doesn't underastand the mechanics of tanking - or has other considerations (loot distribution, etc). The biggest decision you have to make when choosing a tank class is deciding on your play style. Oh, and if you want to look at a fuzzy butt all day. A tank's view is pretty limited as it is. I reccomend choosing something delicious.