Defensive target was amazing, it made so many new mechanics available. Buffing people, healing people based on combo attacks. So good.
Also paved the way for newer games to have this concept, like Warhammer Online. It's actually a significantly major change to how tab targeting MMOs work in all aspects. However, it's an amazing one especially for PVP MMOs.
I really hope WoW someday adopts that in their UI by default. It's a huge QOL feature that I am unsure as to why it hasn't been implemented. Imagine being able to track a focus target defensively and offensively without having to keybind for both of those targets by name or index.