1+2. what about the janrain engage plugin: http://wordpress.org/extend/plugins/rpx/
3. subscribe to comments plugin mentioned by @chip bennet
4. chris pearson discusses flat, but referenced comments : http://www.pearsonified.com/2007/11/professional_stylish_comments_for_blogs.php
i’m not aware of any automated way to implement this, but i think it is a great idea
5. unsure
