I started linkjs[1] so I could create an extensible inbox/social app that supports multiple services and interfaces. The idea is to build tool environments around communication, much like the environments we already have around processing (e.g. the unix shell).
Email will be one of the services the comm environment integrates, so it's more of an extension than a replacement.
For most users that just need to communicate regarding a specific item it is much quicker to comment on that and that alone with tools like http://postfrenzy.com.
Having to deal with threads and multiple topics in single email messages for most users often results in a degradation of the conversation, quickly.
