I don't remember D2 ever having diminishing returns from bosses, though my memory may be faulty. Their primary method was to just temp ban you if you did too many runs per hour.
However, I'm kinda bummed that they're dropping the focus on boss runs. In D2, the Act bosses not only had higher chances of loot drops, but the fights were infinitely more interesting than the random boss packs. The random packs could be challenging at times...but that's usually only if their buffs were waaaay unbalanced. Then, they were usually more annoying than anything else...and heck, there were so many mob types that were simply annoying rather than fun to fight. If they manage to make the random boss packs fun in D3, my opinion might change. If I had to guess, though, I'd say D3 will have it's fair share of annoying, un-fun mobs.
An MMO game like WoW has to have pretty regular updates with new content to keep people interested. If they can truly make people interested in finding and fighting random boss packs, that takes a lot of pressure off needing new content updates. However, giving people incentive to explore random locations because of the loot spread does NOT equal a game worth exploring...I hope they've manged to do it, but I give them long odds.