How about the legacy of arcane spell failure for armor use? When 4e came out, I thought that ridiculosity had ended, but no, it rears its stupid head in the 5e playtest too ... From a balance perspective, I can sorta kinda vaguely understand it (but can't understand why they would then allow workarounds), but there is no in-game reason for it that can't get logicked away in two seconds.
Ooh, and adventuring being better practice for wizardry than day-in, day-out study of the arcane in a well-equipped laboratory!
And all 2e rangers being specialized in two-weapon fighting! Because there's nothing better for protecting a forest than a sword-and-dagger combo.
Finally: the Deck of Many Things. /thread