"What's wrong with DNT? 8 bytes on each http request is probably as efficient as you're going to get"
Technically probably nothing at least on the protocol level. I would assume that the big question is what should fall under the DNT umbrella. Is it ok to include DNT users in your site-wide web analytics stats, or should it just apply to the massively cross-domain tracking methods that advertising networks, Facebook and Google use?
Then of course there's the question where to draw the line on hosted web analytics solutions such as pretty much all of the reasonably priced ones. Technically they can be used to track users across massive amounts of domains, but as far as the site owner is concerned they're simply used to analyse traffic within the site.
In my case I would (as I do now thanks to adblock) opt-in to web analytics tools because they're generally used to my benefit (as well as the site owner's of course) by improving the sites I visit. But on the other hand I'd opt-out of the massively spread tracking methods such as the Facebook "Like" buttons that you see everywhere, because that kind of tracking is absolutely useless to me (since I'm not a Facebook shareholder) and frankly more than worrying in terms of my privacy.
So if DNT is just a check box, should it work like my current setup, should it block all cross-site tracking methods or should it simply block all tracking including any server-side statistics that have a unique ID? And if you provide options in the DNT configuration what should those options be?