The good thing about the old subject line of an email is that you usually can tell whether it's something you need to look at now or save for later, move to another folder, or delete. You can also quickly see whether you are the main target or just CC'd.
Speed reading can also assist in processing email. Sometimes you just know whether you need to read something thoroughly, or can glance over.
Speed reading, as you suggest, would not be a good solution to finding mission-critical information. As far as I know, no collaboration tool exists that could handle the complexity that we were dealing with. We tried quite a few, and pushed SharePoint to its limit.