Keep from D3:
No trading outside of group present when item dropped
(also, treasure goblins as a concept)
Avoid from D3:
Only 1 thing? That is tough.
Endless paragon/progression.
(Also the cube/crafting/kadela, basically everything about itemization, lack of character build customization, RoS combat design, power creep, solo vs. group balancing, free-speccing & armory, lack of meaningful survival bonus, GRifts/endgame, and well, most things)
Like from D4:
Each class having a special class system (Sorc enhancements, Rogue 3 clan choices etc.).
(also Rune word system, potentially, as well as respecs having a, supposedly, meaningful cost)
Dislike from D4:
Again tough to pick only one, but one candidate could be
Shared overworld seems very problematic for having a meaningful endgame design
(other than that, most things about D4 just seems somewhat weak. Not unsalvageable, but also in serious need of improvement, such as; itemization, skill trees/upgrades, attributes, skill diversity, high dmg scaling, end-game potentially having gear-scaling (albeit not mentioned since 2019), no offline support, and much more)