Many people love the way that GMail treats emails as conversations, and soon you will have the ability to do that with Cocomment. This service is currently by invitation only, and I was fortunate enough to be invited. Once I receive invitations I will be posting them here.
Currently it seems like it will be a very strong service. It is easy to keep track of the places that you make comments and now you will only have to check one place to see if you have received a reply from someone else. This reminds me very much of the conversation feature that can be found in GMail.