The reason TERA is so paramount is because it's basically a 3rd person beat' em up with MMO elements and features.
That means it's not a boring snoozefest of me vs. some mob or even better, a PvP player, just spamming abilities until one of us goes down. Every ability is skillshot. Every move matters. Tab targetting is boring and so isn't the combat that comes with it. This brings a whole new dynamic to combat.
This does multiple things, but the most obvious is grinding/farming. These things are no longer tedious and boring. It's so much fun just USING the combat features.
I'm not even gonna go into all the end-game goodies like GvG, Area Control, WvWvW and end-game politics that involve guild leaders and cross server options.
This is just the surface of TERA.
Then again, it's just like any other mmo right?