There's a lot to reply to here, so I'll try to keep it as short as possible. To clearly state my position, I'm in favor of opt out. My reasoning is that the majority of installs should auto-upgrade, and also the sites that should NOT auto-upgrade are also the sites that tend to have a person running or developing them that is capable of adding something to wp-config to opt-out.

Having said that, I think there's excessive concern in some of the comments here. People seem to think this might happen soon. Otto said it perfectly:
We’re a loooong way away from this being possible

When it DOES happen we'll do it right. However, it's going to take time to figure out what "right" is. I see this as another "the media area needs updating" kind of issue. We know it needs to be done, but it needs to be done right and that means we need to take time to plan.

I'm also a little confused as to why Matt got a personal call out here. As far as I can tell he's not trying to force you to do anything. This kind of thing has been discussed by the developers on and off for quite some time. At some point when a good solution is envisioned a couple of developers will try to code it up. Then several more will tweak it until it seems functional. It will end up in core and several more developers will test it, break it, fix it, etc. Whenever this eventually happens there will be quite a few people that were part of it. Matt won't have "forced" this feature in, he won't have "forced" you to use it (there's certainly going to be a way to opt-out), and honestly he doesn't even "force" you to use WordPress!

In the end, if you don't want automatic upgrades then opt out. Those that need them most are the people that don't know it, so for them it should be turned on by default.