Basiclly it works like this.
All priest types have "direct heals" that instantly restore heal, at the cost of power.
Each priest type also has a better power/healing ratio heal spell, that is class specific.
Cleric types get reactive heals. It's a short buff, that proc's a heal everytime the tank is struck.
Shaman types get protective wards. Another short buff, that prevents a set amount of damage whenever the tank is struck. These wards have an effective armor class of ZERO. Less than a mage.
Druid types get regenerations. Once again, another buff that heals over time. It's like a free heal each tick.
Each of these types is best suited to be partners with one of the other tank classes.
Clerics work best with warriors.
Warriors have high AC, low evasion. This means that each time the warrior gets hit, he takes less damage, and the reactive heal procs a heal to even further reduce the effective damage taken. Crusader self wards prevent reactive heals from going off, as does the high avoidance of a bruiser. This is where they myth of "they can't tank" comes from. People assume that clerics are the best, and if they can't heal them, then they suck.
Druids work best with crusaders.
Crusaders have good AC, low evasion, and have self wards. The means they get hit as often as a warrior, but don't take any damage until the ward is depleted, after which they take slightly more damage. The wards are short, but can be used after any damage is taken by the crusader, to buy time for the regen to actively replenish their health.
Shamans work best with brawlers.
Brawlers have low AC, high evasion. Since wards have no armor class, and are uneffected by the tank armor class, the tank is basicly invincable for a set duration, either time or number of hits. Because brawlers dodge more, they extend the period of time the ward lasts, letting them be invincable for a longer period of time. The warrior gets hit far more, so the wards get torn apart too fast, as does the crusader. The crusader has self wards, but only one ward a time will absorb damage, while the other ward just burns through it's duration.
Make no mistake, these are not in any way accidental. The EQ2 dev staff wanted to give an assortment of classes, in each of the primary roles. In order to keep the classes from being the same, they altered the way they did things. They tailored each tank and healer combo to work well with each other. Over all they did a good job.
We just need to debunk the myths that have carried over from EQ1 about which class is which. Warriors and Clerics were the ultimate in EQ1, and now everybody just naturally assumes they still are. They two are the no-skill classes, very simple and easy, even a beginner can play it decently. So people see this combo work well, and work well consistently. They assume it's the best.
The other combos are equally good, if not better, in my opinion. They just take more skill, and thus, more people screw them up and make the classes look bad.