The default human form of organisation is feudal: power structures are fluid and made of personal relationships, and there are no rules, only traditions.
This changes with the introduction of the res publica 1, the public matter. Areas of life considered public gain specific rules that are clearer and stricter than the traditions of private matters. This introduces a distinction between public and private speech. Publishing some utterance, formally associating yourself with it, is now different from saying it in private.
For example, if I'm in my bedroom with my girlfriend and say to her "I really hate Bob and want to kill him", then that's, well, not nice, but clearly private speech. My girlfriend will be weirded out and tell me not to talk such nonsense, but that's about it. If on the other hand, I stand on a soapbox in the town square shouting the same thing, or write letters to newspapers detailing my murder plots, I'd expect to get a visit from the police!
Between the extremes of the bedroom and the soapbox, there is a de-facto grey area of semi-private speech: things said while in public but not to the public. If I am walking down the road with a friend and say something to them, that is not "public speech", even though I am in physically located "in public". If I'm at a party with friends and we're discussing some topic, we are not making public statements on that matter. (I recall an angry drunken night just after the UK phone hacking scandal broke where I definitely said some things I would not want to be on the record.)
This is important enough that it comes with a formal implementation, the Chatham House Rule. Participants of a meeting held under Chatham House Rule are allowed to reveal what was said at the meeting, but not who said it. This is extremely useful, for example, at government policy meetings, where public servants can talk to politicians much more freely.
Google Glass wants to destroy this all. It turns private speech outside the bedroom into public speech. Words said at a party, or walking down the street, become potentially public. Of course, people have always been able to quote others' private speech, but a recording turns an allegation into something more substantial. This changed calculus of risks causes self-censorship. I would never talk politics near a Glass, knowing that my statements can be easily recorded, published, framed, de-contextualised, and disseminated without my knowledge or consent. The problem is not the recording itself, it's the threat of recording, the forced change in culture that makes private speech public.
A lot of ideas considered universally right these days were originally crazy, radical, dangerous — the sort of thing you could lose your job over. Enlightenment ideas of equality and democracy and rational thought were greatly accelerated by the appearance of coffee shops, exactly because these were semi-private places ideal for in-public private speech. Democracy, women's suffrage, the abolition of slavery — popularised through private speech in public.
Radical ideas cannot always take root in the open: the price to pay for speaking of them to the public is too high — which is why freedom of association is so important to a functioning democracy, and why totalitarian regimes restrict it, isolating political opponents and preventing organised resistance. A totalitarian regime wants to make all aspects of human life public.
Private speech is at the foundation of political progress. Glass wants to turn all speech public. It is totalitarian. If you have any political beliefs you would be uncomfortable signing your name under but hope to realise one day, Google Glass is your enemy.
1 This is where the word republic comes from. History and linguistics are fun!