I don't see how the build diversity has changed at all unless your speaking about ranks for abilities/runes of which everyone would be seeking and acquiring the highest rank of both. Which would inevitably lead to the same amount of abilities with the same amount of runes/variations that will currently be in the game.
And World of Warcraft only lets you alter abilities through specific talents (many abilities cannot be altered regardless) whereas Diablo 3 is letting you choose a variant for each ability.
I can see losing the joy of searching for the runestones and giving you more purpose to the loot grind, but there are pros and cons to both paths. I think I prefer the one that is currently slated in patch 13 to be honest. And I like the new rune graphics. :/
Well, I guess a better way to say it would be to imagine that there being no +skill affix in D2 as well as no skill points. So, no way to invest in individual skills. That does decrease diversity to me. With the rank system, anything higher than 4 was meant to be increasingly rare. So let's say you got a lvl5 rune and a bunch of 4s. You'd have to choose which skill to slot to be at lvl 5. Now it doesn't matter.