I'm generally happy with Legendaries this expansion. I only have 3, but they're good enough. I also don't play at a level where they really matter, so having or not having the Best-in-Slot ones isn't super important to me.
However, I think for a lot of people who are more hardcore, the Legendary system didn't really work. In particular, I don't think the concept of tailoring your build to match your Legendaries really took off.
Perhaps the problem was that the Legendaries weren't strong enough. For example, if you look at Diablo 3, set bonuses are pretty insane. If you're wearing a set which buffs an ability, that buff is on the order of a 1000% or more. I don't think such a system--where getting a Legendary forced you to build your character around it--would really fly in WoW.
I think a system that fits WoW better would be something with a little bit of randomness, but also add in control and effort.
I would suggest a scheme where the Legendary drop rate was about one per week, but the item level started at 800 or so. The player could use Obliterum to upgrade the Legendary to the item level cap.
This scheme would get the dedicated player all Legendaries reasonably quickly, but they would have to devote time or money into upgrading the Legendaries they want to use. Using Obliterum as the upgrade material would also help out crafters, giving them incentive and a market for their wares.