I sorta play, I finished the campaign a while back and have been farming T3 but the endgame balance is pretty bad right now. With season starting in 11 days I'll probably only hop on to find altars since those carry over.
It feels like in D3, the game was balanced around average player builds being strong, whereas in D4 it feels like it's balanced around strong builds being average. That along with the gold sink, tedium of actually respeccing, power locked behind paragon glyphs, etc. it isn't super fun to keep grinding. In D3 I could use literally any build I wanted and still farm T16, in D4 I can use 1 of 2 builds and it's still dangerous.
I like that D4 is dangerous at higher levels. At least for now when the game is only a month old. It is infinitely better than the campaign where nothing was ever difficult and the first boss you fought was just as hard as the final boss of the game.
They made a deliberate decision to slow down the combat and make it a little more tactical. You are not meant to just run in and pull 3 elite packs and then curbstomp them instantly. The best experiences I'm having right now are when I'm doing a nightmare dungeon that is at a tier that is just within/at the boundary of what I can accomplish. The combat gets tense and frenetic and there's still a chance to die if you make a mistake with your position or cooldown usage or even if the game just fucks you with a bad combo of stuns/freezes/explosions or something like that. In contrast I find that doing regular dungeons (to unlock aspects) is boring as shit and I absolutely do not find it engaging. At level 74 currently I am not even bothered about my final endgame build. That will only start to matter after level 85 or so when 800+ drops become more common. Till then I'm using suboptimal gear and gems (for example, my weapons have lifesteal instead of crit damage that everyone else uses) and it's fine.
That said, powercreep will happen. They will introduce more legendaries, more uniques, higher world tiers and better gems so I expect other broken builds will emerge. The screen clearing power that you had in D3 will return.
I have some other issues with the game which I am baffled got through into the final release. Mostly quality of life stuff really. How did nobody in the entire dev team spanning years of work not realize that manually having to refund 150 paragon points one at a time is insanely bad design? How did they ship it like that? How did they not realize that bag and stash space is far too low? You currently cannot go through a single dungeon without filling up your bags even when you ignore blues and whites. Was this not an issue for them when they were testing the game? Did they not feel the need to collect loot while playing to see that bags are too small? How is it that all the bags have 3 rows of items but aspects only have 2 rows? There are no labels for aspects and all of them look the same so finding the one you want is so painful. Honestly, there's a whole bunch of UI problems in the game and especially with mouse+keyboard that they should have done better with.
The other aspect I don't like is that it feels like they didn't stop to think which part of the repetition would be fun. When you're making an ARPG and you know that seasonal resets will happen you should always ask yourself if repeating something is actually fun for players. I'm talking about the renown system of course, but it applies to everything in the design going all the way to even things like unskippable cutscenes. It might okay the first time but nobody wants to repeat it the 2nd, 3rd, 4th or 15th time. So for renown their original plan was just "do everything all over again" before they got so much pushback and now they're letting fog of war and altars of lilith persist but it's still not enough. Having to repeat side quests will suck because of how uninspired most of them are and how so many start from random drops you're supposed to find around the world. Having to repeat many (or all) dungeons again to unlock aspects will feel like a burden. And Blizzard should understand this. Mark my words, a day will come when aspect unlocks will be account wide and a one time job. Blizzard doesn't even know this now but it is a change they will make. Until then players will have to suffer through a very tedious part of the game in every season.
The itemization could use a bit of tuning too but I'm not actually sure what needs to be done. Currently the most common way to get gear is to find a yellow ancestral item with 3 good rolls and 1 bad one, and then reroll the bad one to something good. And when you have a good yellow after doing that you imprint an aspect and get your legendary. And it's good that they've given you a way to make legendary items which aren't totally dependent on actual legendary drops but at the same time the only thing you're ever really farming for now are uniques. Most legendary drops you get now will not beat your jerry rigged yellow so that feels a bit bad. Not sure how this can be made better when the aspect system exists and any interesting legendary property they introduce can always be imprinted onto non legendary items.
Another issue I have, and this might be more of a personal opinion, is that leveling is currently too slow and the season is starting too quickly. Seasons were always going to be a thing but I think they should have waited maybe 3 months to let people actually do everything in the game before starting new journeys. To at least get one character to max level and spend some time farming on that character to 'complete' their build in terms of item drops and all. And I understand this totally depends on how much time one is able to put into the game. I'm at 74 and I do not expect that I will get to 100 before season 1 starts, let alone 'finish' my desired builds. At the same time there are probably people with 3 level 100s who have run out of stuff to do and would lose interest without the new seasonal chase. But I do suspect that a large majority of people will not finish their first character and many will probably drop them because they know they can't do it so might as well wait for the new season. And tied to this is the leveling speed which, IMO, is very slow. So again, this is an issue that can be mitigated if you are able to spend large amounts of time in the game and/or have friends who are always around to level up with. They addressed this to an extent with the nightmare dungeon XP buffs recently but I don't feel like it's enough. It's especially a problem below level 50. If you have friends who can carry you through capstones as soon as you roll an alt then it'll take 30-40 minutes to reach level 50. If you don't then you will be stuck doing that part of the game for so many hours. There is no gem of ease to help you either.
D3 got lots of quality of life updates by the end and while I can understand not having everything that game had, it should have been something of a baseline for D4.
Anyway, all that said, I still like the game a lot. They've nailed the combat feedback which is the single most important aspect of an ARPG and what you spend 95% of the time doing. The combat animations, sound and physics work perfectly together to make the combat feel really great. It's something many other games of this genre can't seem to get right. Everything else surrounding this is really well done too... the art, the music, the production values and voice acting and everything was top notch. And I expect that a lot of this friction with the design issues and UI management will be smoothed out in the coming months.