The problem WoW had/has was that dps players damage scaled much higher than tanks. The nature of the gear curve means it eventually gets to a place where one tier is a huge leap so if a player is 15 ilevels behind it caused them to loose aggro. No one wants to wait five minutes so a tank can gain enough threat to stave off your fun buttons.
Blizzard redefined tanks with what they called active mitigation - tanks had to do stuff other than keep threat to stay alive. It was a model blood DKs had used since they were introduced.
Personally I'm in the school of thought that tanks and healers should not do significant dps and if they have spare time then bosses need to be tuned better or do more, or more frequent damage.
The problem with abandoning the Trinity is that you get boring encounters. GW2 started out like that and has introduced it slightly.
Games without the trinity have weak boss damage and become roll out of danger games. That got really boring in GW2 base game; hopefully it's improved in recent ex pacs.
Besides, being a tank is more about knowing what you're doing and how to treat boss mechanics. In particular, knowing when to dodge an AoE and return while it's visual effects are going off (ability permitting!) so you dont move the boss and kill peoples' combos etc.
It's astonishing how many 'tanks' take the boss for a walks. Don't get me started on tanks that go to dps and take a bosses cleave to the group...
