People seem to misconstrue that Any form of role is set in stone via a Trinity system. If you have the ability to mitigate damage, you are a tank, if you can heal yourself and your allies, your a healer, if you can stop your opponent from attacking, your a CCer/Controller.
But that isnt necessarily true, look at ret paladins for instance, retribution was meant to be a DPS spec, yet for a good amount of time, Ret was able to do great offhealing that could rival that of a healer. But just because he could heal well, doesnt mean he is a Healer specced character.
The trinity system literally means that there will be a person who specifically heals, a person who specifically mitigates the damage of the enemy from reaching others, and there will be one person who does damage and prevents enemies from doing excessive damage to others by controlling them.
ergo, this means the trinity provides people will three roles to fill, Tank, healer, and DPS/CC.
The problem with this sytem is that these roles literally bind the player into ONLY fulfilling one role as their best form of contribution to the team, that means a healer can only heal well, but cannot dps or tank well. A tank can mitigate damage well, but cannot necessarily heal well or dps well. A DPS can do damage, but cannot heal or mitigate damage like the tank or healer.
^ This is the trinity system at work, you cannot deviate from this format, period. If you chose a role (lets say dps for this example), you are stuck with it (unless you respec to another one), you cannot do anything but this role well, and while you may contribute a heal or two, or tank the mob a little bit, you are not expected to do anything else but DPS.
In Guild Wars 2, this is not the case. The healing system in GW 2 prevents any one person from being able to heal others continuously, instead, everyone has a handful of self-heals, each of them may have different effects, some may protect you, some others. Some may even heal others if they are close enough to you, but you cannot sit there behind everyone else and heal them for the duration of the fight.
The same applies to Tanking, as we know so far, no class can withstand continual punishment, each class has about the same amount of limit as to how much punishment they can take before going down, and since there is no official healer who can keep them up since the healing system prevents it, no one can sit there and take a beating for an excessive amount of time during a fight.
That leaves the DPS/Controll system, from what we know, every class has control abilities, while some may be more effective than others, every class has the potential to shut down an opponent for a small amount of time, leaving an open window for them to help others or fight another opponent altogether. Since there is no tank or healer however, DPS must find a way to keep themselves alive while dealing with their opponent who is attacking them, this may involve kiting, CC, creating a barrier between them, or even letting another player take a few hits. Because of this, ALL players must cooperate with eachother and coordinate wisely in order to overcome a powerful enemy, because no single person can tank or heal continuously throughout the duration of a fight.
Therefore, if no one can tank an entire fight, and no one can heal an entire fight, that means there cannot be anyone assigned to those roles and be good at them, therefore, there is no Trinity System.