My least favorite trope, I just avoid immediately, so it's not a huge problem for me. It's when an author wants to write a child/adult story, but they want to make it surprising or less appalling or something, so they make the child look like an adult, or the adult an immature failure at life, in a big reversal. Far worse to me is the totally played straight romances between young girls and adult men all over shoujo manga right now.
Something that turns me off of a manga I might otherwise find okay is the homogeneous art style that you find on "cute girls doing cute things" applied to yuri. Half the time that includes just a small amount of subtext and no real yuri, so it basically is those shows in effect. On the polar opposite side, when sex scenes are drawn with disgusting amounts of sweat, it completely fails to be hot to me and just becomes funny.