@aral This kind of depends, I know this is about tech but here's a prime example: The UK made organ donation opt-out, because most people won't make the effort to actively opt-in. Would you disagree that was a bad idea?
Then there's things that people like which depend on consent from many users: Full text search for example. In cases like this, I like opt-either where you must pick your preference.
Most people are not informed about choices (or even look!), what we need is better information.