You won't be farming bosses in Diablo 3
In a recent blue post on the Battle.net forums, Technical Designer for Diablo 3 Wyatt Cheng went into some detail as to how loot will work in the upcoming hack-n-slash title supposedly set for an April release. According to the post, you will not be farming bosses in this game for the best loot, which was the standard for end game in Diablo 2. Rather, you will be fighting rare and champion packs of mobs that have random enchantments on them - multiple per group. Check out the post below:
You will not be farming bosses. Bosses won't drop the best loot, they won't even drop really great loot. Part of Inferno and our intent with getting people out into the world and hunting and killing lots of different things is putting the best loot on rare and champion packs, and the great thing about rare and champion packs is they have random affixes. They're like a box of chocolates. Murderous, snarling, blood-soaked chocolates. You're not going up against a boss where you know "Build A" is the best way to minmax against it because it has abilities and resistances X, Y, and Z. What is the best build vs. an "Arcane Enchanted, Teleporter, Frozen, Knockback" skeleton pack? Got that figured out? Cause it's not going to be the best against the next pack you come across, and you're going to want to kill that one just as much.
You might have a specialized build that is super strong against some of these things, and not against others. Your focus is going to be on the balance between taking on all of these possibilities and surviving, and it's that balance that makes for a ton of interesting options and variance.
The one question mark for a lot of people, and maybe even us, is what stops someone from seeing a pack, backing out (or dying) and swapping out to be better equipped to handle it? We agree that shouldn't be the best way to play, but know it's something we can solve pretty easily, even if it's just making the swapping cooldown longer in later difficulties.
In any case, his point was that you could absolutely make the best build against one type of enemy, and that build could completely fail against another. It's not D2 where you pump all your points into one ability, we're going for some depth in our combat, but it's your choice of tools (and there are a lot of them) that will define your character versus another.