I much prefer to see one message per row/line of data along with the timestamp of when it happened.

A threaded top-level view feature is probably already on the wish list.

I'm not sure whether I'd vote it up or down. As it is, what we have is a lot like Hacker News, which is demonstrably useful.

Still, I wouldn't stand in the way of making this an option. mail-archive.com is also useful, and that's how they display a list they archive, by default.

how do I know which things I've read already?

Currently, the best way I know of is to enable email notifications. You then just read each new message via email, visiting the web interface only when you need to post something or search for old posts.

Features for changing the visual style of read messages have already been brought up several times. Someone's just got to get around to implementing it, I'm sure.