Double Experience for each level 60.

[YGSLlew]

The new tribes scale with level and higher ranks offer more exp

You can start them at lvl 50

Indeed. They are scaling based on your Rank with the beast tribe and your current level. IIRC, at max rank and lv 50 they start at about 70k exp per quest and scale up to roughly 100k per quest at lv59. Given that all 6 dailies you can do don't even take half an hour, it's fast exp for literally no effort. And still more fun than spamming fates.