Warhammer Online introduces a new system for targeting creatures and players for spells and abilities. It is interesting to compare the Warhammer Online system to World of Warcraft. WoW actually has multiple targeting systems, each serving a slightly different purpose.
World of Warcraft Basic - Primary Mode
In the default UI (no mods, no macros), there is one target frame. You choose your target, and press the ability, and the game tries to cast it on the current target. So to cast a spell on a new target generally requires 2 operations:
1. Select new target
2. Press spell button - Spell is cast on current target
This method has the advantage that it is very obvious and intuitive. It has the disadvantage that changing targets is fairly costly in terms of operations. Additionally, switching between a hostile and a friendly target will turn off your auto-attack.
World of Warcraft Basic - Secondary Mode
In the default UI, there is a secondary targeting mode. If you have a hostile target selected, and you cast a friendly spell such as a heal, the cursor will change to a glowing hand. You can then click the frame of a friendly player, and the spell will be cast on that player without changing your target.
1. Press spell button
2. Select friendly frame - Spell is cast on friend, target is not changed
The advantage here is that your current target never changes, your auto-attack does not turn off. Offensive actions don't require a target switch, and only cost 1 operation. However, this method does not seem that popular among players, and is probably not very intuitive. Additionally, even Blizzard occasionally forgets about this mode. For example, spells that can affect both allies and enemies, such as Holy Shock, will not work properly, automatically defaulting to hitting the hostile target. Another annoyance is that all your friendly spells get "greyed out" as unusable, even though you can still cast them.
Warhammer Online seems to have two targets: an offensive target and a defensive target. Selecting a friendly player changes your defensive target, and selecting a hostile changes your offensive target. If you cast a hostile spell, it hits the offensive target. A friendly spell hits the defensive target.
In terms of operations, this is slightly different depending on what you are doing. If you are mostly attacking creature or mostly healing friendlies, then it effectively costs 2 operations or so per target switch. However, when you alternate between healing and attacking, it only requires 1 operation. In the best case scenario, you only have yourself and one enemy, and you never need to switch targets. I would imagine that this targeting system promotes attacking and defending at the same time. If there's no extra cost, you might as well throw out a DoT if you have time.
As well, this system is pretty intuitive, while only being slightly more complex than WoW Basic - Primary. The idea that offensive spells are cast on your offensive target, and defensive spells are cast on your defensive target just makes sense.
World of Warcraft Advanced - Focus
In WoW, you can declare one target your "focus" with the /focus command. Right now, you generally need macros or mods to work with the focus target properly, but in WotLK the focus target is being built into the default UI.
Having a focus target allows you to cast certain spells at your focus, instead of your main target. This is especially useful for crowd control, as you can automatically renew a crowd control spell without switching targets. Essentially, casting a spell on the focus target only costs 1 operation, while other targets cost 2 operations.
The focus target is also open-ended. A DPS can juggle two hostile targets. A healer can juggle two friendly targets. However, one of the targets is fixed, as it takes a bit of effort to switch focuses.
Focus, while very powerful, is not exactly intuitive, and takes some getting used to.
World of Warcraft Advanced - Mouseover/Click-casting
In WoW, you can also macro your spells to target the frame or mob that your mouse is hovering over. This essentially reduces the cost of all your spells to 1 operation. This is primarily used by healers, as they have to switch targets often. However, DPS and tanks do occasionally have uses for mouseover targeting. In particular, warriors will use a mouseover Sunder macro to build threat on crowd-controlled mobs. (Sunder doesn't do damage to mobs, and using mouseover ensures that you don't auto-attack the sheep.)
Click-casting is where you automatically cast a spell without changing targets if you right-click a frame. You generally need a mod such as Clique. Click-casting is pretty similar to mouseover targeting in effect, but you're limited by the number of mouse buttons you have.
These techniques reduces operation cost to the minimum. However, they are fairly unintuitive, and require extra setup on the part of the player.
In a lot of ways, the basic WoW targeting system promotes focusing on one activity at a time. If you're healing, you switch between friendly targets and heal. If you're dealing damage, you stay on your target and burn them down.
Warhammer Online seems to promote both offensive and defense. The dual-targeting system promotes using both abilities that help your allies and hurt your enemies. This doesn't just apply to healers. Any class could get and use helpful abilities, and still be able to concentrate on the hostile mob. Additionally, it's still very user-friendly and easy to understand.
However, both Focus and mouseover targeting offer much more control, and reduces the cost in operations, to the experienced WoW player. Mouseover targeting effectively obsoletes the dual-targeting system for healing, and Focus allows you to alternate between two hostile targets. Still, these techniques are not exactly user-friendly, and generally require the player to have a fair bit of experience in the game before she can master them.