The way the new buff works for end-game (this is only active at level 60):
Here’s how it currently works internally: Rare and Champion packs already have great loot on them. By killing a Rare or Champion pack, not only do you get their loot, but you’ll also receive a buff granting you increased magic find and gold find. However, if you change a skill, skill rune, passive, or leave the game, the buff disappears. As an extra reward, if you kill a boss while this buff is active, you’ll receive extra loot drops from that boss.
And some other info from a small Q&A they did with fansites, it wasn’t posted in the Blizzard forums:
When you say extra loot, do you mean, extra loot. Or do you mean extra loot that is on par with what elite/champs drop. I remember from the article before, bosses do not drop gear that is as good as champs/elites.
They will drop gear as good as champions and elites with this buff (because you have to kill champions and elites to get it, which was the point). With the buff active you’re guaranteed one extra item drop from the boss, and it’s likely that it’ll be 1 item per stack of the buff. Up to X stacks. We don’t know how many it’ll be, probably a handful.
How many stacks will there be if any?
We’re not totally sure but probably a number you can count on one hand.
Do you lose the buff when you die?
It lasts through death currently, and we like it. I know some people will also want it to be a punishment for death, but repair costs are already extremely impactful.
Does the buff splits between players?
Like someone joins after you’ve been killing and they have less stacks? It’s a direct increase to your MF/GF, so it uses the same mechanics.
Do you lose it on disconnect?
There’s a small grace period, similar to WoW where you can get back into the game before your character is actually logged out (ie a timeout), but yes if your character times out you would lose the buff.
What is the Time limit on the stack, if any?
We’re messing with times from about 15 minutes to an hour. We don’t know yet.
Do Stacks get removed after a boss?
We don’t really think it needs to. Your focus is still going to be seeking out champions and rares. If you want to get full stacks, kill a boss, and then try to rush to kill another boss before the buff falls off… have fun? It’s not going to be the most lucrative way to play, so it’s likely a non-issue hypothetical.
So, level 60 characters will start a game to farm uber loot, and choose the spec they want to use for that game. As they run around killing Elite groups, they get the bonus, so they get more uber loot for bosses during that game. Eventually, all those will be dead, or they’ll be out of time. At that point, they can start another game, and respec fully before they start killing again, if they want to try something else out.
It’s a fairly elegant system, especially since it’s set up as all bonuses, no penalties. It allows for full customization between game sessions, but not for switching specs to deal with whatever type of Champion/Rare packs are in that game, since there’s no way to know ahead of time. Of course, if people really want to change specs mid-game, they can; they just won’t get as good of drops for doing so, during level 60 end-game content.