With the exception of the iPad, the adoption of every major consumer technology platform was driven by a handful of distinct, killer applications:
Desktop: Excel, later Word, and then the web
Cellphones: voice + text
MP3 players: music
There have always been niche applications, such as Photoshop, or AutoCAD, for specific verticals. But those verticals never drove adoption of the technology platform by 25% or 50% of the population. There have always been just a few killer apps that drove adoption.
One might recall Apple's series of commercials, "there's an app for that", over the past few years and say that the killer-app theory is wrong. But that's not quite right: people bought phones for voice and text; Apple highlighted the apps as a way to differentiate themselves relative to Android.
But what about the iPad? How did Apple drive adoption without a single unique iPad specific application upon launch? The iPad handled all of the common computing tasks, but was cheaper and easier to use. Additionally, the iPad drove touch based computing. Large touch canvases are powerful and intuitive. They are inherently natural to use and manipulate, especially relative to a mouse and keyboard. No baby every figured out how to use a laptop on their own, but there're dozens of videos of babies using iPads with 0 training. Touch, simplicity, and better/cheaper/faster were the defining characteristics of the iPad. The iPad was successful because it made more computing more accessible to the masses across virtually every dimension.
So what about Glass? What's going to be the killer app that drives adoption by 25% or 50% of the population? There isn't, and won't be one. The problem with Glass is that it's not actually better than the status quo for the vast majority of the general public. Glass competes with the smartphone, and it's materially worse at all of the major smartphone functions: voice, text, Facebook, Maps, email, and web browsing. Unlike the iPad, the user experience is quite unnatural. Given the screen size constraints, the absolutely wretched awfulness of the trackpad, and the timeline, the entire UX is simply unintuitive. I've personally watched at least 500 people put on Glass for the first time across a variety of social contexts. No more than 10% were able to understand Glass's UI without guidance.
Moreover, marketing Glass is an uphill battle. It's easy for Apple to make commercials of someone's hand swiping away on the iPad. But there's no way to virtually simulate Glass. You have to put Glass on your face and play with it to understand. And it's not immediately obvious how to use it. Sure, Google tries to provide virtual simulations, but these kinds of simulations aren't nearly as accessible as a hand swiping across a beautiful touchscreen.
For Google's sake, I hope Glass succeeds as a consumer product, I just don't see how it will. Glass needs a killer app, and it's not there.