Maybe this is my teacher half talking but I've found that a little patience and explanation is incredibly effective.
In 28 levels I've seen a lot of players who just don't understand the group dynamics -- they are either new to MMOs or they think this is WOW. Simply saying don't do that or please do this (without being a jerk), has remedied the situation every time.
I believe a lot of it is due to the way tanking and healing mechanics work in this game, at least early on. Tanks are still fairly squishy and aren't built for holding aggro or taking damage from more than 1-2 mobs. Also, healers pay a heavy price for spamming heals. I don't have 80k mana, I can deplete my entire energy bar in <5 casts, and every other cast lowers my regen rate.
that's why I target what I do. Like I said in my first post. Weak enemies are under estimated in large numbers. The 3 normal mobs together are doing more damage than that one strong one. So as dps I feel I should kill off the weak guys instead of the tank target to lessen the numbers against us. Sometimes that "target the tank target at all times!!!!" Crap doesn't work. Especially in this game. I think we have established in this thread that despite the similarities to wow, combat mechanics are not like wow. Not every fight is tank and spank.