Long ago the group tag/tap was removed and if your faction was fighting a World Boss you could jump in right away and get your share of the loot. Unfortunately it was frustrating to see the other faction fighting a boss you were waiting for. There were only two options: You either wait for them to kill it and wait for the nest spawn or kill you had to kill them all and reset the boss.
A hotfix, that was considered a bug at first, changed this and now both factions, Horde and Alliance, can participate into taking down a world boss. I’m pretty sure that it will be a bit messy, but at least you’ll get your loot share.
The confirmation of this hotfix came from twitter
— Watcher (@WatcherDev) April 2, 2014