I like how Blizzard gave add-on developers the reins on UI development.
They built a really cool game, but the UI was kinda crap since launch; add-ons made it useable.
I think the UI addon devs took it too far, but Blizzard's class design has been so diabolical that honestly a lot of this was needed. Like the game just wasn't playable due to rotational issues and things like DR tracking.