Diablo 2's end game was all about Diablo himself. Toughest boss in the game with the best drops. D2 LOD was no different with Baal. I personally don't find it rewarding WHAT so ever to attempt to kill Diablo at all after killing him once to progress the difficulty stage. This is a huge disappointment to me and I'm sure the community.
Diablo's fight in D3 I personally think was the most challenging out of the Diablo games. A much longer battle. This battle with grouped players also brings a slight challenge of facing your companions shadow clones can sometimes be very tough and the ability to continually pounding Diablo as he's dying is currently pointless but has potential to be something rewarding. (reach a spec. amount of damage while bashing to increase drop rate)
Gathering 5 stacks in ACT 4 can sometimes be tough and most of the time you must face "The betrayer" ( :D ) in order to gather the complete 5 stacks to face Diablo. With the time invested in gathering 5 stacks and the sometimes lengthy battle of Diablo, it doesn't feel like facing this battle is worth anyone's time.
I believe by changing this, it may freshin' up end game and bring a bit of familiarity to the game. Personally I feel Diablo's drops need to be better and this should only be applied to partied games or public games with multiple players. Pushing this into games where players can do it solo will make this easier and less rewarding then to push it with party games and public games. It makes Diablo himself harder to face and will allow players to stop and share what they found.
We have keys to farm for, BOA hunting, Act 3 XP runs... Now I think it's time to give us Diablo runs. A true Diablo end game is Diablo himself.
After reading the recent post of itemization. I think they are going in the right direction but I think viewing this post from them would give them a better idea of what to do. I really dislike the idea of the first time you fight a boss is when you get the best chance for great loot. Reason being is what if on that first fight you got bad luck? This would mean your screwed for future boss fights.
Allowing us to take on Diablo with a much greater chance for great drops in group play is a great way to give us new additions to end game and the ability to play as a group like the classic Diablo 2 style that I think majority of us miss.