A little late to the party here, but openwebmail supports all of your feature requests except IMAP. Openwebmail does not require suid or root if you are only running it for a single user's mailbox.
In a multi-user environment I don't think there is any other way to allow read/write access to the spools of all users besides suid. If anyone knows a way...?
Regarding gmail, conversation view is nice. However, ads are not and so does the idea of not owning my data. When its on my hard drive I don't have to worry about anyone else rummaging through it to include me in their demographic. Gmail also does not include a full featured addressbook or calendar, which is essential for me.