TSW had dungeons like GW2 should have had. If they added a dedicated tank and healer to GW2 I think it'd be a lot better PvE-wise.
Healer yes, tank no, in my opinion.
I think ArenaNet was always slightly off the mark when it came to their stance on the 'trinity'. Being able to heal extremely effectively - and more importantly, to pro-actively
protect effectively - has almost totally beneficial effects on the design of a game. It allows the designers much more freedom to give interesting, powerful abilities to the AI enemies, it provides an alternate path to being good and skilled at the game, and it provides an automatic (if limited) safeguard against class imbalance, by making sure that no matter how powerful a damage-dealing class is, there's going to be at least one person in the group who cannot be replaced that way.
The
only downside to having effective, powerful healing in the game is that it then forces every group to have (depending on group size) at least one or two healers in the party.
Tanking, on the other hand, actively removes depth from the game. Tanking goes hand-in-hand with manipulable 'aggro' systems, and those are among the absolute worst, most shallow mechanics you can put into your game, by letting the players control their opponents as well as their own characters. It's like letting one person play both sides of the chess board - it's just not very interesting. Enemy AI needs to be free to attack the
most opportune, vulnerable targets in the players' party -
not forced to attack the
least opportune target. And needless to say, it shares the downside of the healer, in that once you implement it,
every group is forced to have one (and often,
exactly one).
TSW minimizes how boring the trinity system gets because A) In group play,each dungeon is essentially a boss rush, and each boss has unique mechanics that help to mask some of the core flaws of the game, and B) In solo/duo play, the freeform, classless, levelless skill and equipment system means that the trinity is no longer in play at all.
In general, though, you want a system like Guild Wars 1, where you have powerful healing and protection, multifaceted classes that can handle a number of roles ranging from damage to control to ally support, and absolutely no formal aggro system - you can make one character into a super hard to kill juggernaut, but if you want the enemies to focus on that player, you have to actively make that player the most opportune target that the enemies have.