Blizzard sits behind the term, "RNG" but anyone who has played this game excessively knows that certain drops have a much lower chance to drop. The odds are stacked against us, the itemization is A LOT more like the lottery than it is random. There are several major changes to the itemization in D3 that differs from D2, two of them I consider to be the root of the cause.
ILvl: while also in D2, this would determine whether a certain item could drop or not, there was another factor involved called "TC" or treasure class. In order for certain affixes or legendaries to drop the monster had to have a certain lvl and treasure class. For example Pindleskin at one point could drop anything in the game, later Blizzard went back and lowered his treasure class since he was too easy to farm. Now ilvl is another ball bouncing around in the lottery machine.
Affixes: not the actual affixes themselves although I plan on getting into them, but how they used to work.Before you could get 3 prefixes and 3 suffixes on an item. This has been changed that an item can get 6 prefixes, this is why you never find any good rares. There are 120+ affixes total, (some don't apply to every item) in most cases you need either four or five prefixes in order to be good. On top of that, each affix then has tiers from its min to max value. So in order to get a really good item you need to win the lottery.
With the update to legendaries, gear has gone towards a more "LoD" approach than "Classic" and while people seem to prefer that, you guys did a really bad job with it, I'm sorry to say that. Some of the updates are detrimental to the itemization of the game, or in your case the end game. Manticore is one of the worst offenders, but Lacuni's are by far the worst along with Skorn, Meepo, Witching Hour, Inna's pants, and the offhands.
Stacking ONE legendary of each type with one amazing unique ability followed by most of the affixes needed is a huge imbalance. You now have set the bar for most rares to low end of these legendaries. Ideally you should have one unique affix, one prefix and one suffix, allowing for three more random mods. This would of forced you to spread the love across all the legendaries making most of them useful instead of one clear winner. You also wouldn't have set the bar so low for rares.
We're going to have to live with these legendary changes, the current issue with itemization affects rares more than anything. You have addressed the ilvl part of the itemization and I feel it was an improvement. You need to take a look at how item's are generated and revamp the prefix/suffix list. This change shouldn't affect any important gear, you would possibly make a collector item out of a few blues, but I doubt it.