The only thing that bothers me about character design is when older characters are made to look like children or children are made to look like adults, especially where romance is involved. I really like and recommend the comic 'The God In The Field' on this site, for example- it's a beautiful comic and the story is one we all know- but I simply cannot follow it because it really creeps me out.
I guess something in me needs to know that where there's romance involved it's two adults, two teens, or an adult and a legal teen (like really legal, I'll have none of that barely a woman/man shit) or whatever-- so long as it's laid out and I know what I'm looking at.
Also really dislike it when girls and boys (like 16/17 years) are made to look like 30?? What's with that?? If you have a character who looks like an adult and behaves like a bratty child it's extremely confusing.