I have an idea for a plugin for which I can't find the functionality in any other plugins or combination of plugins.
This plugin would add two check-boxes next to the comment fields on a blog. One would be "Notify me when replies are posted". This would then send an email to their commenting email address if/when someone replied to the comments on that post (I know this functionality is available in plugins out there, but none seem to be compatible with the current version of WP).
The other checkbox would be "Subscribe me to [blog name here]", this would subscribe them via email to that blog. So for example, I use Feedburner which includes an email subscription option. Rather than having to type their email in, then type a code, they could just click this box as they post their comment.
Its just good usability, and the kind of plugin I'm sure a lot of people would love to have on their blog. Please reply here if you have the ability and would like to create this plugin. I'll be the first to send you a donation if you create it as I described, and do a good clean job of it.