This has been a global tendency over the last years: to get rid of Spam and other unsolicited, unwanted or unneeded emails, ISPs have tried to classify email senders and sort them into 3 categories:
1 - Spam and unsolicited emails : Usually refused by the server
2 - Unwanted emails : Usually sent to the junk folder
3 - Regular mail : Delivered to the inbox.
The thing with this system was that it was based on a statistical analysis of several elements (complaints, hardbounces, email quality, average open rates,...) out of the total number of their users you were targetting.
Gmail is now rolling out their beta version of Priority Inbox. This system will continue to work along side the old one (meaning Gmail will keep filtering incoming email based on statistical data, but now, they will sort the emails automatically for their users:
Gmail will analyze how you react to a certain type of emails and will sort it accordingly in your inbox.
This sorting will have two levels : important and unimportant.
According to what we know, this analysis will be based on how often you read the emails you receive from a given sender and how often you reply.
The "funny" thing here is that this system will make it almost impossible for email marketing emails to reach the priority inbox, the reason for this is that it's quite unlikely that any of your clients will open each and every email you send to them and even less likely that they will ever reply to one.
At the end of the day a "regular" inbox will probably be cut in half : the personal emails up and the newsletters down.
I expect quite a drop on open rates for Gmail addresses in the first weeks / months after the roll out, time for the users to get used to this new display and to get the habit of checking the bottom list for your communications, but I also expect that this system will lead the way to new forms of one 2 one spam filters that will no longer be at the ISP level but at the user level.
This will have several implications:
First, it will emphasize the need to clean up your database on a regular basis since your inactive users will very likely no longer receive any of your emails.
Second, the deliverability management will no longer rely solely on ISPs but will have to fine tune and focus on how users react to the broadcasts.
On the user side, I'm pretty confident this is a huge leap towards clean inboxes and value added email communication.
On the ESP side... good luck, have fun!