The leader can choose one (not more) of the members of the team, and that member gets a great skill, which is replaed his Elite skill. Every class has it's own skill, and each owns in it's own way
For example:Warrior: Fury. For 20 seconds your attack speed and movement are increased by 33%. When you hit an enemy, he is Crippled and Dazed for 10 seconds.
Ranger: Spirit of Gods. All teammembers gain 100 maximum Health and 10 maximum Energy, skill recharge 25% faster. This spirit dies after 60 seconds.
Mesmer: Confusing Blast. All surounded enemies are interuped and Dazed for 15 seconds.
Elemetalist: Burninate! All surounding peasents, eeeh.. Enemies are burned for 5 secons.
Necromancer: Minion Army. All surounding corpses spawn a level 17 Minion.
Monk: Uberpacifism. For 20 seconds none of your teammembers can be attacked.

Seriously though, everybody would give the monk the uberskill and fights would get boring since nobody would be able to attack the other.