After having got 3 characters to 20 this weekend (don't think about how long that took, I forbid you!), I've got a few thoughts on Diablo IV in a totally random order.
1) They went too heavy on the quests. @Whizbang Dustyboots
is right re: "a modern World of Warcraft", but the quests aren't actually as advanced or fun as WoW quests often are now, and there are just endless numbers of them. You don't have to do them, if you don't, they going to sit there acting like you should. They should have done half as many with twice as much effort put into them, frankly (and commensurately greater rewards), given that the game scales. Also the rewards being pretty much all boring, generic, and unspecified gear is not helpful - you can't even go "Oh this gives me something nice, I'll do it!", you just have to hope.
It's okay for an ARPG to be about killing stuff, Blizzard.
2) Scaling is a little excessive. It's definitely working, and well, but it's also managing to make it feel like you're not really getting much better, overall. Not ideal, imho.
3) The UI is awful. I hate it. It's ugly as sin (in a bad way), and anti-immersive in a way I've never seen before in an ARPG, so that's kind of an achievement. Plus very clearly designed for consoles - loads of stuff requires a key-press where it should obviously have a clickable element on PC (or the clickable element takes some finding - but sometimes it doesn't even exist!). It also makes the game feel like a mobile game.
4) There's a bit too much pure, unaltered Diablo 3 DNA in Diablo 4 generally, and not enough of Diablo 2, Path of Exile, and so on. It's almost like 4E's Essentials as a reaction to 4E. This manifests in a variety of different ways, like the very regularly-sized packs of monsters, the hideously neon-blue skeletons the Necro has (please for god's sake change it Blizzard, it doesn't remotely fit the new aesthetic), the crafting/gambling systems, and so on. Diablo 3 wasn't terrible, but it's sad to see none of these elements was innovated on in the way D3 innovated on D2, because it's been 11 years, and they could have done with innovating on.
5) The gameplay flows well and is very smooth, and most classes feel pretty interesting and different to play - it's certainly better at lower levels than any preceding Diablo game, and arguably better than most Diablo-clones in this regard.
6) There's a serious oversight in not giving us a "preview" mode for the skills - I know we can change them fairly easily at first, but it's dumb, and I feel like Diablo 3 had it (would have to check). It's especially annoying as a lot of skills do NOT work like their description would seem to imply.
7) Some of the systems/ideas feel definitely half-baked. For example, the Rogue take on Exploit Weakness requires way too much build-up and precision for a tiny, brief reward (ohhh 3s of free energy, likely at a time when it's totally unneeded!).
8) There are way too many abilities which are "just rubbish" unless you buy the enhancements AND get a piece of gear which makes them do something extra. That's not the right way to balance things, frankly.
9) The classes are hilariously, insanely, unbalanced at these levels. Maybe that'll be fine in the end, but it doesn't feel like it, given the Druid is approximately 10x harder to play than the Rogue which is itself like 3x harder to play than the Necro. You expect a bit of this in ARPGs, especially whilst levelling - pet classes are usually very easy to play - but the Necro also just totally destroys stuff.
10) The game is incredibly stable and good at alt-tabbing. I was astonished. That's not normally something you see in a beta. This is game built by people who definitely expect you to be alt-tabbing and stuff.
11) The classes are interesting and diverse - but I have to assume there's absolute ton of later-game or endgame gear which enables various builds not evident from the skill tree, because otherwise the number of builds is very, very limited (I've kind of heard there is, though).
12) This is possibly the most twitchy ARPG I've ever played - even including FPS ARPGs like Destiny 2 and Borderlands - I was surprised by that. I mean, Path of Exile has a lot of builds which require some serious reflexes, but it's not as keen on punishing you for not instantly reacting to oddly-timed boss animations with tiny reaction times - there are bosses here where you have to dodge in a 0.5s window which isn't necessarily as a telegraphed as I'd like, for example. It's also very much designed knowing you have that dodge - it isn't a bonus - it's a necessity, and if you blow it at the wrong time, you're going to get spanked. I've got mixed feelings about this. It keeps you awake but it also means I'm not sure it'll be as fun to play mindlessly as some ARPGs.
13) It runs astonishingly well for how detailed a lot of the graphics are. The characters particularly look incredible and wow it's wild to have fairly diverse outfits and so on again in an ARPG, something not present in most (including D3 and PoE). Aesthetically, as noted, I feel like it owes just a little bit too much to D3 (which made some serious aesthetic errors and I'm not talking about "not dark enough" or whatever) and the UI is godawful, but the characters and world look really, really good. Hopefully more areas will make it even better - kind of already looking forward to expansions which might take us to jungle or something (as my impression was that there isn't one here).
14) These bloody accents. I know, I'm British, and most of the characters are either British-accented or sound like British people doing bad Russian accents, but oooof. No. The female Rogue and Necro just have terrible voice-choices. They're not so noisy it's a huge problem but I honestly cringe every time the femRogue speaks - maybe I need to try a male one.
15) Enjoying how diverse the characters look body-shape-wise, even though it's triggering the hell out of the usual suspects (especially the misogynists) on the internet, but I really feel like they should have had more faces for everyone - indeed I thought in early vertical slices they showed like eight faces per gender per class. Four very similar ones ain't quite cutting it.
Overall, whilst I have a lot of questions and quasi-complaints, I am a big fan of ARPGs and the Diablo series, and I had a lot of fun, so I think I'll probably be getting Diablo IV when it comes out, so long as I hear they've listened to at least some feedback from the beta - particularly re: the ghastly UI - but it does seem like most people hate or at least dislike the UI so maybe Blizzard will.