Haven't read the entire thread-apologies for dups. Heavy dose of opinion here.
Critical Threats/Hits ala 3.x
Cyclic Initiative and Abilities/Spells that are tied to "until the start/end of your next turn" and the like
Level Drain/Level Loss (though Undead need something to make them more frightening than they have been in recent editions ).
Balanced Encounter building. They can't get it right after 3 editions anyway because it's too difficult to judge individual players/groups/styles, so why bake in all the mechanics for DMs to worry/spend too much time on?
"Off" spell levels- Level 5 Wizards get 3rd level spells. Either go 4E (break out character levels) or 13th Age/DW (condense character levels-my pref).
Vancian/Quasi Vancian Casting, I love the books, love the fictional reasoning- in play it's just a resource management accounting game that takes all the magic out of magic, and encourages metagame thinking.
XP for monsters/gold
Spellcaster levels for Monsters (back to 4E style powers)