First, the non-traditional currency system. There's no gold; instead you have different types of "orbs" of varying rarity. Vendors have different exchange rates for some of them, but the system is generally difficult to grok, especially for a newcomer. To make matters even more confusing, orbs themselves are useful, e.g. they can enhance item stats, upgrade items to higher quality, etc. and you need to be careful to not accidentally continue using orbs yourself when you'd be much better off trading them.
Which brings me to trade. There's no centralized auction house - instead you need to rely on a third-party website where items are listed by other players. The process of listing them is done via an in-game API I think - but then once you find an item you want, you need to whisper that player, then go meet them at their hideout and manually do a trade. Just an absolutely atrocious system overall.
The third one is the zoom level. The game feels like it plays at 800p, or maybe 1024p, and there's no way to zoom out further. Unacceptable for a game that is two decades more recent than Diablo 2, imo.
