The main difference I can see with labels (at least for power users) is that the mails get filtered automatically by Google in these categories. I believe that most of us here on HN won't see this feature as revolutionary but it might help the less tech-savvy ones.
For quite some time, I've been happy with GMail because I set up a system through which each email is ultimately stored in one of my predefined labels. I wrote an article about it [1]. It's both simple to setup and easy to use. And although I don't receive that many emails, I believe it can scale quite easily.
I have a similar system, but yours does take things a step further. I'll definitely feel like wasting an hour sometime revising my own setup :)
That said, I think you pretty much hit the nail on the head. If the automatic filtering is done well, it will be a killer feature for normal users.
From glancing over her shoulder, I know my girlfriend has hundreds, if not thousands of unread mails, most of which would probably end up in the promotions tab.
I'm positive this change will make it much easier for her to check her email at a glance.
I actually like the tabs idea, but at the moment I find it useless. If there was an option to show my custom labels (instead of the predefined ones) it would be an awesome addition.
outlook.com has a similar feature, but it seems far less obtrusive and a bit more useful. Having a "Shipping Notifications" category for instance has been really handy. My gut is the outlook.com implementation is more user friendly in general. But I'm happy to see two big email services now competing to add useful features (even if execution might be flawed at times).
For quite some time, I've been happy with GMail because I set up a system through which each email is ultimately stored in one of my predefined labels. I wrote an article about it [1]. It's both simple to setup and easy to use. And although I don't receive that many emails, I believe it can scale quite easily.
[1] http://jgthms.com/organized-gmail-inbox.html