I realize this post has been confusing for a lot of people, and I can understand why. Being a little more familiar with the source material, I still have a hard time articulating just what Hera is without discussing other upcoming [alleged] changes to Android that we will talk about at a later date.
In retrospect, showing Hera before some of the other changes we're digging through may not have been the best idea. Hera is more of a concept than anything - it's not a design language, it's not a replacement for Holo or a new set of design guidelines, (though it may be based on some new design guidelines) and it's not a new application.
Hera appears to be a new layer in the Android OS that will manage, at least to start, many of your Google-related tasks and content. The point of Hera is to provide a way to quickly and easily access the things that matter to you across a range of devices. When we said it would bridge Chrome and Android, the point there is that Hera will be the universal interface whose purpose is to simplify and streamline the bridge from desktop, to Chromebook, to tablet, to phone.
Imagine you were just having a Hangouts conversation on your Chromebook. You leave the house and realize "Oh, I forgot to tell Steve I'm going to be late tonight." Instead of opening up the Hangouts app, you pop open the Hera multitasking interface, and right there in front of you as a card (probably one of the few most recent, depending on what last Hera-associated task you did) would be that conversation so you could quickly reply.
Or, imagine you're looking up directions to a new restaurant on your tablet. You're getting ready for dinner, and you head out the door, get to your car, and realize you've forgotten the name (or address, whatever). You pop open the Hera multitasking interface and one of the last items you'll see will be maps, with the directions you had open last on your tablet.
Hera is about streamlining, simplifying, and unifying the way you use your devices - not the way they look - and making using them a "just works" experience. To put it as Eric Schmidt did, roughly, it's about answering a question before you even realized you had it. It's about unifying the information that matters to you, or could matter, in the quickest, most intuitive way possible.
Instead of opening an app and looking for the right conversation or address in your history bar, Hera will just know (hopefully) what you're looking for. And because it's just an HTML5 layer acting as an Chromium instance, there won't be the traditional mess of intents and compatibility with local, native apps - Hera is not meant to provide a fully featured experience in these activities, but to be a simple and [hopefully] hard to break tool that eases the pain of doing those activities across devices and platforms.
I hope that clears it up a bit, though do realize I am extrapolating based on my understanding of Hera, and some of the information we have. This is all still a rumor, but maybe knowing the what I speculate to be the philosophy behind Hera can resolve some of the confusion people have had about it.