What things aren't explained and don't make sense? Besides the cartoonish inventory space. That I'll grant wholeheartedly, even if I still think it's funny.
Why does my bow damage pop up multiple damage numbers? Why does my offhand attack have a lower chance to hit? Why can everyone disengage as a bonus action? Why are the ranges of some weapons and spells dramatically reduced? Why does Hunter's Mark consume the buff granted by Guiding Bolt? That's just off the top of my head.
Let me be clear, I am
loving this game. But the parts that provide the most frustration are the parts where I'm confronted with a change from 5e that makes my experience worse.