Thanks to everyone for voting, it really will help a lot.
Inventory system: People clearly like the current inventory system more than the classic’s, weight based inventory system. Of course there’s much more to this (like keybinding) but the weight and inventory slots are the main reason here.
Atmosphere: People mostly like the dark, immersive atmosphere of the classic more than 3.x.
Attachment system: This one surprised me the most. I expected people to love the ability to put scopes on literally everything. Apparently not.
Zombies: People like how zombies can chase them everywhere, and they are much harder because the player isn’t able to stun the zombies. Overall, people prefer the “hardness” of 2.x zombies than 3.x zombies.
Cartography System: In classic, you had to find a map yourself and in order to see where you were, you had to carry the map in your inventory (which, in 2.x, every inventory slot is important to have) and also had to hold it in your hand, so you couldn’t just easily switch between weapons and maps. At some point, to save inventory space, you wouldn’t even carry it with yourself and had to know where you were at all times. Of course, in 2.x, this did not really matter because there was only one map (ignoring monolith) that is PEI. In 3.x, all you have to do is find a chart/gps and press M. People were very indecisive here, looks like.
Melee weapons: I thought people would like 2.x melee weapons more. In 2.x, melee weapons actually mattered. Especially the sledgehammer and the katana. @Doy_Man summed it up well here:
Ranged weapons: People undoubtebly like the variety of ranged weapons in 3.x. Not just that, but ranged combat was also a lot better in 3.x. No need to argue here.
Base Building: Majority of people like how easy it is to build a base in 3.x. In classic, it was really, really time consuming, arguably unneccessarily complicated. This is connected to the crafting in some ways.
Alright, i sorted the results. Please let me know if i am wrong in some parts. Thanks to everyone who voted.