Gmail's categories are a pretty reasonable starting place: primary, social, updates, forums, promotions.
All reasonably identifiable by word choices, and often at extremely different urgency / priority levels.
But I would personally also have an "interesting" category for newsletters and whatnot that are more likely to get a full read (newsblur has a weak version of this for my RSS, it's nice). And an "urgent" category that notifies more visibly than others. "Recruiters" is also periodically useful - a bunch of cold emails filled with buzzwords, should be identifiable a mile away.
But underneath it all it's mostly that it confuses me that there's so little ability to experiment here. Shared always-updating spam filters makes sense, but there's a lot of similar things people could try unique to their needs that are just totally unsupported. I don't want better global categorization (gmail), I want personalization. Email is weirdly non-personal in the vast majority of systems, and I suspect it's part of why people burn out on it and switch to a never-ending churn of apps that are a better fit for one piece or another.