The goblins have been around for as long as I can remember in the diablo franchise with d3 adding the very rare type where you could follow one into the portal. These and the cow level have a been a stable to the franchise. As far as I've heard no one has found the cow level in d4 yet but there are some pretty strong leads at this time. If anything I would say those games copied Diablo but don't quote me on that.
I don't remember when I saw the goblin/thief for the first time, and I can't tell who was first in this case, but many other things Blizzard took from Korean or Japanese MMO that were released before (like pre-2005). It's like when someone did something that is selling then why not to use it. There is no law against it. WoW and some of their other games are mainly popular in the US/EU and Blizzard marketing is pushing it on the "western market". Asian games are a bit different and have more demanding players (mainly because of many more grind hours or higher difficulty levels). It causes that even if one or another side copy anything, then most players won't ever notice that. I remember that when I was playing other games, then some players were mentioning that Blizzard copied many things from other popular titles (and had some good examples, but it was years ago and I don't remember most of that).
Diablo is another thing. When the first part was released, barely any game was similar. It was something fresh and somehow new (even though was looking a bit like Baldur's Gate then felt different in any way). Back then, Blizzard was very small, and I feel they just went for the money after the second part. I remember a lot has changed and they grew pretty fast. Everything else was only about money and gamers see that clearly. Especially Wow was a hard push for the money since release. It had the most expensive subscriptions, but people were paying, so why not. Only Final Fantasy Online was more expensive when was released some time later.
Just my thoughts about Blizzard. Diablo is the only thing I like from their games. Probably because there is nothing similar on the market and most new games are too colorful and candy-looking.
You might try to find an item with or extract/imprint the legendary skill that gives you unstoppable when damaged while stunned. It makes a huge difference for me since as a melee I take a lot of stun hits. I keep that and the aoe from my hammer of ancients on me most of the time while the rest change from time to time. I even add them to yellows if needed for the current item build.
I have some unique items, but most are useless. Like armor is great, but with skills that I'm not using. I'm using only an amulet as it gives additional aoe dmg. Yesterday I got a unique scythe but it's also useless for me.
On Saturday I moved to 4 difficulty level as on the 3rd many things were too easy, and I wanted better gear. Dungeons are much harder, especially when I moved at 68lvl and I still had some 45lvl items. I still have one 45lvl and I'm 72lvl now. I don't know why but I get almost only garbage from events and bosses now. Yesterday I passed helltide 3 times, clearing most chests and I had almost only yellow items from them. The same from the tree.
I went summons+blood so I run with 13 summons and deal aoe blood dmg that is healing me. I have additional +blood nova +aoe with every nova. I only have problems to regenerate essence fast enough so I could spam it all the time. I also took all special paragon nodes for summons. I still feel it could be much better, but I can finally see every upgrade as I die less and clear faster in time.
I'm not sure how long I will play D4 as it feels like I did almost everything already and almost only grinding for better items left. I still have some side quests and stuff that appears at higher levels, but I feel that not much will surprise me anymore. We will see, I hope there are many things I haven't seen yet, and will be some larger updates.