I fully support automatic updates for minor releases, without any "opt-in", not even for existing sites. This has been my view for years. Because:

* Minor releases are mostly security related, and fixing some edge cases that broke in the major release

* I firmly believe this is the interest of he vast majority of users, with old and new sites. Having them to "opt in" makes the whole thing not worth it. It would be as to say that there is a danger in this option. Even if WordPress "recommends" it, they may beleive not opting in is still the "safest" choice. That would be "options, not decisions" at it's worst

* WordPress will better ensure the integrity of the downloaded updates.

* WordPress releases, minor and major, are extremely stable and extremely (too?) backwards compatible. I have been running lot of sites for 9 years now, and I can't remember one single plugin that broke completely because of upgrading WordPress (may have forgot things that happened in 2004-2006). What I remember is a (very badly coded) plugin that broke, and slightly corrupted a site, because the host silently upgraded PHP, back in 2008.

* It will be possible to "opt out" of the automatic upgrades through a constant. That's all we need no know, as developers. Plugins that sets this will be there before 3.7.1 is out.

* I have installed and used the Automatic Updater plugin on all my client sites now for a couple of months and it's just amazing

If WordPress later wants to automatically upgrade even to the next major release without using "opt in", I will also support that. When it comes to plugins and themes, some day, I will recommend to have a end user "opt out" for each individual plugin and the active theme, as long as the plugin/theme author hasn't actively prevented a user "opt out".

Such individual "opt-out" possibility is the only thing I miss in the current Automatic Updater plugin.