Dialogue that is utterly nonsensical and not at all reflective of how people talk in normal conversation. I'm sure this is often a translation issue, but this is so noticeable when exposition is shoehorned into dialogue.
For VNs it's nice when one is built in, and if one is required because there's substantially different story branches off of different choices, then that's nice design, too. Moreover if those choices aren't OBVIOUSLY intuitive as to render a walkthrough moot, but not so obtuse as to seem...
It's really tough to say. I flip flop on this every time I see new stuff printed about the available tools. Might destroy the world, might be revolutionary in technical fields, who knows at this point.