This is actually a tough one, because I have several.
That item management won't get better but worse as the game progresses is at the top of my list.
That the difficulty settings will either be super easy or ridiculously hard - "Oh, but we have you 5e RAW difficulty setting like you asked. You need multiple miracles from God to play through it, but we gave it to you."
That reactions will suck in the end.
Oh! No. This is the biggest.
That the story ending will suck and make no sense and I'll hate all the characters by the time the game ends.
