That's the problem with D3 loot imo, you've got rares with uninteresting stats which are fairly common and legendaries with awesome stuff which is insanely rare but nothing in the middle. To make a comparison in loot models lets compare a sorc going through loot in D2 and then in D3
Rare > Cheap Runeword > Spirit Shroud > Vmagi > Tal Armor > CoH. Each adding a new dimension to your character with a rare just being random stats to the runeword giving you a huge boost and then spirit shroud getting you that +1 all skills, vmagi finally some resistances and tal armor so you start building that set and getting that MF rolling before you enter the realms of the godly and put on that insane runeword you've been building. Now without any sort of cheats it still took a very long time to get to the end, but the entire journey was interesting.
Rare > Rare with 20 more int > Rare with 40 more int > Rare with 60 more int > Legendary. This is how I see it now, you're just constantly hunting for that end-game item with nothing interesting until you get there because the game lacks mid-game loot.
The end game loot needs to be rare but the more common loot that you find up until that point needs to feel like a massive upgrade in itself so that you don't get bored before you get to the end, not just "yay I got 20 more int" that doesn't feel like an upgrade, even if they are getting stronger it makes the player feel like there not improving, not finding anything because theirs nothing substantial about it, it's just boring.
I believe this is tied in to the fact that in d2 you never reached max level before you finished gearing your character. I mean, you start gearing your character while leveling up but when you reach like level 85, you can pretty much equip all of the items in game but you still have 14 more levels to go and the last 7 are insanely hard to get without botting.
I agree that D2 had more interesting uniques than D3 but that might be related to the nostalgia factor more than we would like to think. The difference of level requirement between the Spirit Shroud and the Vipermagi armors is only 1 iirc. level 28 and 29. When you look at that measily one level in relation to levels 80-99 that's really nothing. So the progress is not that well built in into the game more like how lucky you get and what items you got for free from random people and friends... or just traded for them.
What is really interesting in my opinion is that a perfect, upgraded, socketed Vipermagi, can actually be a very viable end game item especially if there no bots and dupes. In D3 it's really impossible to get a low level legendary and turn it into a viable end game item which is quite sad in my opinion. This aspect of gear customization should have been covered by the enchantress.
I also agree with your claim that in D3 it's "Rare > Rare with 20 more int > Rare with 40 more int > Rare with 60 more int > Legendary" but i'm not sure that this is neccesarily a bad thing. When i found rare bracers that had 50 more int and 2% more CC than what i already had, i was really excited. If you are getting upgrades, the game is interesting. Your statement might be more right in the environment of D3 vanilla mainly because of the auction house, but i believe that the expansion will partially solve that and make loot more interesting by giving us more constant upgrades and not just gold to buy stuff off of the AH with.
If a second expansion comes with systems similar to the D2 runewords, then that issue will be diminished even further. I think Blizzard are on the right path with ROS in general.
Maybe legendaries will be somehow tiered like runewords in D2. If we were to find about 10 equivalents to the Spirit Shroud before finding one Skin of the Vipermagi, there would be enough "middle ground", right?
And btw, referencing lvl reqs of D2 is pointless imo. Sure it was great to equip Vipermagi at 29 while leveling up, but when do we ever lvl up chars in D3? I'd say, 95+% of the time in D3 is spent at max lvl anyway
From what they have been saying I believe the Legendaries are in Tiers they are just not presented that way. But they have said you have to go to higher difficulties to be able to get the better legendaries.
So it's more like:
Basic Rare -> Rare with better stats -> Rare with good stats and maybe some specific skill bonus to your build -> Basic Legendary -> Better Legendary or Set -> OP Legendary or Set.
And about the higher level legendaries dropping in lower levels. I'm not sure if it works that way. I thought they said they will now drop from their current level in Vanilla and UP (they never said anything about down). I don't see why they should/could not do that though.
There is no need for low-mid good loot anymore because we don't create dozens of character for different build like in D2. You make 1 char of a class, get it to max lvl and that's it. Now i'm thinking about all those low lvl uniques that we never got to use because when found them we were 60 or they just simply sucked for the lvl requirements and progression on where we should of been able to use them :/
IIRC, they said something about legendaries being able to drop in any difficulty or level, for example, that a Leoric Signet could drop at level 60, and it would have level 60 stats.
So, I imagine the same could happen the other way around, a level 30 Skorn, why not.
I'm pretty sure that it has been confirmed that every item has a base level and that it can drop on that level and anything above it, but not under it. So if the Skorn's base level is 30 for an example, you'd be able to find a Skorn only when you are level 30-70.