GW2 is a very proactive game because your role is not shaped by what class you choose to play, but what tools you use (i.e weapons!) .
For example, Guild Wars 2 turns stereotypes on their heads as a warrior with a longbow can deal major damage, while a ranger with a greatsword can be very effective.
So the ideas about thinking about melee classes versus ranged are gone out the window. Certain classes can use the same weapon types, but they use them in different ways. A Mesmer who is a cloth wearing spellcaster uses a greatsword to shoot laser beams, making clones and applying buffs. A Guardian uses a greatsword to give supportive spells and offensive damage.
You are using two weapons at any time. At lvl 7 you unlock the ability to weaponswap. By pressing ½ , your skill bar changes as you change to your other weapon.
I like for example on my warrior to wear a greatsword as my main weapon, and then swap it with a rifle (great single target damage) as my secondary weapon. So as I engage the enemiy with skills like bladestorm and 100 blades, I apply bleed effects to all surrounding enemies. Then I switch to my heavy attacks to wear them down. if low on health, I can roll out of combat, swap weapon to my rifle, and pick of the enemies one by one as they are bleeding out.
weapon swaping is very important and half your arsenal. you constantly have to swap weapons. this effectively means that you are playing two roles at once. you are a multiclass.
And since I can choose between a wide arrange of weapons (depending on class) like sword / sword, or sword / mace or perhaps mace / axe, I get all sorts of different abiillities. equipping weapons in the off hand also gives completely different skills. so you get different abillities depending if your sword is in your main or off hand.
On top of all this in group combat, everyone are responsible to heal themselves. everyone gets a heal skill, and everyone can ressurect anyone.
another thing is combos. which is nicely explained here;
http://www.youtube.com/watch?v=QrzwFP1vCag