Some users might want to see new conversations instead of just new messages. So there might be a page with links to individual conversations instead of individual messages. Here's a sample UI: http://movicont.nfshost.com/list-sample.html.
Users will likely use the data for each conversation to determine whether or not to click through to the actual messages.
Suppose the user wants to see the answers to a technical question. S/he might want to use the dates of the messages to determine their relevancy–a question posted back in version 0.0.1 of a project may not have much relevancy if the user is looking for a solution to a similar problem in a current version. Thus, the date can help determine whether or not to click on the link.
The problem then is what type of date the user wants–the first message's date or the last one's? For users who want to catch up on posts because they've been away from the mailing list for a while, they would be interested in the last message's date to determine if the message is 'new' enough for them to need to read.
On the same vein, if the user wants answers to a question s/he may want to check the number of messages in the thread first. If there's only 1 message in the thread, the question likely hasn't been answered yet and thus clicking through is not so useful.
The user may also have seen a good chunk of the messages in a conversation before (perhaps they were inactive during a vacation period) and are only interested in catching up on the more recent activity in the thread. Thus, giving a link to the last message may be useful.
The user may be interested in contributing to a conversation, and thus a reply link should be provided (though it is likely that the user would want to read previous messages before actually contributing, and so a link is more useful in the actual conversation thread display).
The user will want something that will show a general summary of the thread, and the subject is likely to do. See below for more about the subject.
Which subject should be displayed when showing conversations, as there can be multiple subjects within the conversation?
When a user first creates a new conversation, s/he is likely interested in directing the topic to be about X. Thus, the first message's subject has a higher chance of summing up the rest of the conversation. Possible problems: there might be an entirely different subtopic developing after a message with a different subject, though I'm not sure how to address that at this point.
Possibilities include:
Since it was mentioned above that users interested in catching up with the mailing list might be interested in seeing the most recent posts (and thus the conversation thread most recently updated first), it is likely best to sort by last message date.
I sorted the conversations with most recently replied to first, and while I was working on it, I realized that the sort order might've been clearer if the meta-data on each conversation could show those last message dates. Thus, the format I generated looks more like a table with the subject line, date, and email address. Here's the archive I generated.
I ended up showing the first message's subject because the first post usually drives the conversation and thus is most relevant. Possible problems: there might be an entirely different subtopic developing after a message with a different subject, though I'm not sure how to address that at this point.