There are a few main types of resources that drive class abilities in most MMOs. This is a categorization of the ones I've seen.
1. Net-Loss Resource
This is a resource which, at the end of the encounter, is lower than it starts. The classic example of this is mana. Classes with mana start at full, and spend it as the fight progresses.
One of the advantages and disadvantages of mana is that all abilities are available right at the start. As well, there is no innate restriction to using any sequence of abilities, until the entire resource is exhausted. This can cause burst damage in PvP, but it does grant the character maximum flexibility to deal with whatever situation may arise.
The major problem with net-loss resources is that they are very dependent on encounter length. If the encounter is longer than the resource can sustain, the character cannot do anything. If the encounter is shorter, the resource does not constrain the character in any significant way.
Net-Loss resources work best when their rate of consumption depends on something in addition to time.
In my view, Net-Loss is the best resource type for healing, because healing depends on damage done. Additionally, having the most flexibility to deal with unusual situations is very valuable when healing.
2. Cyclical Resource
Cyclical resource are used up and renewed multiple times in an encounter. The cycle length is much smaller than encounter length.
The main advantage here is that total time does not really constrain the class. The class is constrained by ability costs and the rate at which the resource regenerates.
There are two main subtypes here: resources which start at full, and resources which start at empty. There are also systems which use a resource that starts somewhere in between, but those system tend to act like whichever extreme is closer.
Full-phase resources (such as Rogue Energy in WoW) allow characters to take immediate actions when an encounter starts. This gives the player something to do as the fight starts, but offers the possibility of burst damage in PvP.
Zero-phase resources (such as Warrior Rage in WoW) require characters to wait before they can take actions. This usually guarantees that the other side has a chance to do something, but is often not as much fun as taking actions right off the bat.
In my view, zero-phase has the potential to be more strategic, but it depends on how fast the resource is acquired, and how important the trade-off between building or expending the resource is. One example of a strategic zero-phase resource is power points in Wizard 101.
One interesting pattern often used is Two Linked Opposite-Phase Cyclical Resources. One resource starts at full, and as it is used, it generates the other resource which powers different actions. Examples of this in WoW are Rogues, where Energy generates Combo Points, and Death Knights, where Runes generate Runic Power. This patter is used because it is relatively simple, but deep enough to be interesting, and provides gameplay where several abilities are used at different times.
I think that cyclical resources are the best for both tanks and DPS characters. Generally, threat and damage depend more on time than anything else, and the other resource systems tend to be too good, or not good enough, depending on encounter length.
3. Net-Gain Resource
This, I suppose, is more a theoretical resource than anything. I'm including it mostly for completeness purposes. This is a resource which increases over time.
The only resource I can think of that does this is Land (potential mana available for that turn) in Magic: the Gathering. On turn 1, you have access to 1 mana. On turn 2, you have access to 2 mana, and so on.
This resource depends heavily on encounter length. If the encounter is long, the resource ceases to constrain the player at all.
This might actually be an interesting resource type for a PvP game. It would make the early stages of the game very strategic, while the final stages would end in a flurry, ensuring the game does not drag out.