I've mostly always handwaved away concerns people had about Google and privacy as being overblown, and I think the idea behind Glass is pretty cool and potentially limitlessly useful, but the privacy implications of a world where Glass-users are common and all data goes through a single entity are staggering. Particularly since they will impact you even if you aren't explicitly opted in (just being seen/scanned by someone who is wearing the device potentially involves you in it).

A world with even just a fraction of a percent of people actively wearing the devices but with all that real-world data streaming back to The Goog in real-time would make Batman's little cell-phone network scanning thing in The Dark Knight look absolutely quaint in comparison.