I see a lot of UI talk in here. I don't know... I've seen some pretty messy UIs from hardcore raiders (Sco, Slootbag come to mind). Mine is also not the bestest and I know tons of people who click half their "not-so-important" skills, doesn't make them any less of a world rank ~100 player.
For me personally the three things would be:
1. Amount of time invested
2. Awareness and
3. Ability to learn and adapt quickly.