Vanilla Diablo 3 you're only grindan for the sake of it. You're not acquiring better gear so that you can kill a tough opponent easier. I mean I killed Diablo as a level 2 monk and it was the same experience that I could've gotten even if I was level 50. There was no ultimate goal to work for. Everything was too level scaled in the worst possible way.
You did try to play classic Inferno Act 2-4, right? You know, the Inferno in which even nearly bis geared characters got one shot on a regular basis. When it was so retarded that people just died through the game to get Diablo to kill him because any elite pack was spelling doom and even just blues were extremly deadly. Iirc, the classic D3 wasn't that much level scaled the first 2-3 maybe 4 months. They then nerfed Inferno heavily and gradually changed the entire difficulty system.
I never played Diablo 3 on the release day so I didn't get to experience they "joy" of how dumbshit it apparently was. I waited to the game to have a price drop and by then they had toned it down. It was around the time when the RMAH was just being closed down. I got to play a bit with the old loot system and then with the new one. Honestly I don't see why anyone would voluntarily waste their time on the game before the new loot system. It was pure garbage.
You missed out quite a lot, for better or worse. I got to play it for free on my GFs account but gave up after 1-2 months. Inferno act 1 was doable and still somewhat fun but starting act 2 was just frustrating to no end. There was no leveling scaling but they really overdid it with Inferno. Its just no fun playing for dozens of hours to get miniscule upgrades which still let you not enable to play the difficulty with any measure of fluidity and you just continue to die over and over. Worst was for hardcore players, most were literally stuck in act 1 Inferno for months since getting noticeable upgrades was about as likely to happen than winning the lottery 3 times in a row. I do not think any hardcore player managed to get to Diablo.
The changes they made with RoS didn't really improve the game by much, tried it out again for 1-2 months but that D3 team seems to be clueless how to balance a game.
I mean bash PoE as much as you like, but at least it has shit ton of build diversity. D3 has like 1-2 builds per class which work at high level content. At best. I am a sucker for wizard and sorcerer type of classes and wizard in D3 is just an utter mess right now with only 1 viable specc which relies heavily on one set which is boring as fuck to play since you have to stack fire damage to get a super OP DoT and then run further and watch them die, to add insult to injury that DoT from the set deals more damage than your entire skill set.
Haven't touched the game for about a year with a short peak into the PTR to see if there were any noticable buffs made but it was the usual Blizzard dumbfuckery, buff something into OPness and nerf or ignore everything which is still shit.