Gamma dungeons are currently accessible on PTR
No new mechanics (so far anyway) but huge buffs to mob HP (as high as 300%) and smaller buffs to damage. Each player picks a buff at the beginning which pretty much just corresponds directly to their role (armor shattering on melee, haste to allies on damaging spellcast, self-buff to damage on heal and super thorns)
Each boss drops triumph badges. As of now sidereal are not dropping from every boss.