Ok so time for me to give an honest quick assessment on D4. I think that the game during the campaign is fun as fuck. After that, everything falls apart. My final verdict on the current state of the game: 6/10. I'm still playing it, but I rapidly lost all excitement of it after the post game.
Before I start, let's not forget that they nerfed the shit out of barbarian (and a few other class abilities) 1 day after the pre-launch. They knew damn well that some stuff was overtuned, but left it all in without announcing any nerfs because they didn't want to drop a nerf patch pre-release and have people cancel box sales. Really shitty to have people choose builds based on how the game was at release only to get nerfed right after when they had all the data from the closed beta and knew full well how overtuned some things were.
Anyway:
There is a massive amount of "workaround" and "hacks" in the game. For example, group up with a friend, and have him start kiling the boss. Go into the dungeon and b-line to the boss. Kill the boss within 2 minutes of YOU going into the dungeon, and you get the "complete a dungeon within 2 minutes" achievement. Or, better yet, just go into a party of 4 and have everyone solo different dungeons. Everyone will get completion for every single one, without actually having to step foot inside it. Or, what about side quests? Get all the challenges simply being in the group when the quest chain is complete, everyone gets it. There's numerous ways to exploit the grouping system to speed up dungeon reforms, abandoning dungeons, etc to avoid running back. It's all dogshit. While none of it has a huge impact on the game itself, its massive, glaring oversights that should not exist in blizzards flagship title.
The mount system is terrible, and the mount gets caught on every pebble on the ground, grinding you to a screaching halt. Combined with rubberbanding you'll find yourself wasting your mount speed charges constantly because you're standing in place rubberbanding everywhere.
If you want to play solo, you're purposefully fucking yourself. Everything about the game is better in a group. Even the aforementioned getting dungeon completion simply being afk in a group while others do it. Having to run to every dungeon is retarded. Events are very cookie cutter. There's about 20 of these, and they simply repeat. It's basically FATEs in FF14, in diablo form + cellars.
There's a gigantic amount of friction in this game. There's so much running around, so many things are inconvenient for no real purpose. Why are half the strongholds showing a clear open area on the map only to be blocked off and only have 1 entrance that actually works? Why is the tree so far from teh waypoint, and why can't I earn anymore points after 10? Why am I forced to constantly port back in and turn it in? Why do so many towns not have the vendors you need? Why if you want to port to your friends do you have to first go to a waypoint in a major town, then take a portal to your friend? Why the fuck is the portal in the middle of the stash and anvil when the stash in in a fucking house, upstairs, and the anvil is on the opposite side of a building? Why do you have to CONSTANTLY deal with gems cluttering your inventory for no real purpose. They basically pulled a PoE here, making you "feel the burden" of your inventory. It's really stupid. Too much bad loot drops, and putting (Sacred) and (Ancestral) on the name of the item itself is completely lazy. I feel like I deal with full inventory more in D4 than I do in PoE. That says something.
Combat is almost identical for every single class from what I can tell but I havent played every one so I can't say for sure. But every class has a generator and a core skill. Spam generator, use core skill a few times, rinse repeat. There's not a lot of originality here. It's very slow, boring gameplay, ESPECIALLY on a barbarian. Combined with the fact that EVERYTHING in the game scales with your level, you never really feel like your gear is having any real impact on performance outside of build-enabling uniques. Every other upgrade is there to simply keep up with the leveling curve. While the level scaling is neat, the fact you don't really feel more powerful is very bad and poor game design.