A while ago I played around with my gmail. The problem I had was that every email from facebook to Linux ended up in the one and only inbox. The solution which has to get a lot easier is to have user rules that automatically puts emails into different folders (labels).
Remembering how many unread emails you have under each label can be difficult. Here you need some additional solution to notify the user of new emails. Since many people have their email with their internet service provider. I hope this kind of solution becomes somewhat of a standard.