Threaded messages is a feature, but it would be better if new child messages were not hidden in a collapsed parent. People miss incoming child messages if the thread is collapsed. Additionally, the ability to read all the conversation in a single scroll-able thread, which hides repeated (quoted/forwarded) content, would be a nice addition.

Conversation support is the standard of efficient e-mail management these days. The Option of just turning on/off basic support for it is not going to cut it. There needs to be more options and customization. And it needs to be intelligent so it's helpful.