I picked up D4 largely to play it with my old MMO buddies I used to play DAoC and Warhammer Online with and have disliked it far less than I expected. It's got it's set of problems but I'm having fun with it, and I absolutely despised D3.
But I don't know how anyone could claim it's more complex than D2 or has more build diversity that you could use in the end-game. Just the fact that D2 has runewords that grant you abilities from other classes widens the field so much in D2's favor that in my opinion it's not even close, and the recent patches in D2: Resurrected have only made more classes and abilities viable.
D2 has goldfind barbs and magic find barbs that revolve around buffing up your merc to the point they can target farm certain areas meanwhile your character acts more like a sidekick who spams Find Item on their corpses. D2 has all kinds of unique builds due to the strength of it's itemization, such as werebear Sorceress builds, Exploding Arrow builds for the Paladin or my favorite the
Riftsin who is able to tackle any immunity by proccing like 5 different elemental abilities from other classes like Frozen Orb, Flame Wave and Tornado. All viable endgame.
Just the fact that skill damage in D2 is decoupled from your weapon damage will result in more variety and gearing options until the end of time, IMO.
That being said we'll see how D4 pans out in this regard in future seasons and patches. I like the Aspects and Paragon board in concept and I hope in the future they add more interesting ones and tweak what you can do with the board more. I'd like the ability to move where you can put the glyph on some of these boards. Aspects to me are essentially D4's version of runeword abilities and I think their implementation is rather clever, although I wish they were a little more ballsy with some of the 'aspects' that are tied to uniques and made them stronger.
I've played a lot of ARPGs over the years, this past year I've been dabbling in Din's Curse for instance. There are certainly things I dislike about D4, but I think the one advantage it has over D2 is that Nightmare Dungeons are fun, Helltides are a good time and generally it has some enjoyable and repeatable endgame content. Once the Cow level, Chaos Sanctuary and Bhaal become trivial affairs the endgame definitely get stale in D2. I could get into where D4 is lacking but we'll see how things shake out after some more patches.
