The game is mediocre because they focused on the what was up front (graphics) without spending the resources on the back end to create anything of value. Two examples (and yet there is a whole longer list):
Mounts - I can see why they would have this idea. Another object in the game they can pass on items to the shop. Another tool to get younglings to spend money. The actual implementation is wretchedly bad, this after they “fixed” some issues. Now add in that in 2025 they still can not do a map overly or even compass for direction. We are left with travel that is mind numbingly frustrating, awkward, and mostly just horrible. Visual is there of the mount, the implementation of a system of travel that is not frustrating is not there.
Items - In D4 we get blues & yellows dropping up to Torment. They serve purpose only for a short lived time, and no other. They are almost meaningless items. In T1 they become an annoyance if we get one dropping. Most of the game has Legendaries & Ancestrals that are just RNG upon RNG that ultimately are bland. Only value is ability to use an Aspect. The original implimentation was a complete embarrassment of development; I am talking about the “% to damage on a Tuesday, in afternoon, but only when raining” idea of affixes.
Uniques, are for the most part not superior items, mostly rather bland, and often of use because they can sometimes take advantage of a flaw or bug. None truly stand out as “ZOMG look what I found”. We have Mythics and on release they were literally non-existent. Yet even Mythics while having significant impact on this game are still rather bland in many ways. A few of them are of value only for their salvage.
Fanbois hate us looking back to D2, yet D2 was right there for D4 developers to look back on for a system that just worked. Build on it, innovate, evolve. Instead they ignored. In D2 you will have some uniques drop in early stage that will be used for the entire journey into end-game. Yellows (rares) could drop with affixes that far exceeded any item in the game. Some of the most cherished & expensive items ever found were rares and not uniques or runewords.
D2’s blues (magics) actually served more of a role in end-game than during the journey. They were part of a crafting system that was truly crafting. Here again, D4 they have what they want to call crafting, and it is an insult. It is gambling.
I won’t even touch on the whole damage model put into this game other than to say it is a disaster and embarrassment. This explains why a game made 25 years ago still has players, and why a modern new game has so many disappointed. Visually and at a shallow glance we see D4 is something with potential, something to enjoy and have fun in. Deep down it is shallow, empty, and void of any complexity. It only takes a few days to a few weeks before players are just grinded down to the stage of “No Fun”.
p.s. I will also throw in Tempering as a prime example of a system that never should have been implemented the way it was, or worse allowed to exist for this long of a time. This game out during the whole Loot 2.0 which while improving the loot, still about 10% the way there.
M.E.D.I. O.R.C. 
