I've been following the evolution of the Kobo E-reader from the very first e-reader of theirs, although my first post on the topic was after the second generation came out, when they added the wi-fi ability.
The most obvious change is the size. The Aura has a 6.8 inch screen where the Touch and the Glo both have 6 inch screens. As a result, the page is more the size of a hardcover's. So, for those of you that don't like to change the page that often, this e-reader will be able to fit more text per page. Definitely something that I like about it.
Like the Glo, the Aura has the front-lit screen, but they have improved it a bit. There's less bleeding around the edge of the screen, making for a more even lighting. This is the biggest advantage of the Glo and the Aura. I certainly wish my e-readers had this feature. It's perfect for reading on air-planes, especially those overnight flights. I remember with the Kobo Wi-Fi when I was travelling that I'd forgotten to have the light with me, and the provided light was almost useless it was so dim.
Now to get more technical. The screen resolution is 1440 by 1080, which is supposed to be one of the best on the market for this type of e-reader. Going along with that is the increase in the controls you have over how the text appears. Font sizes, margins, line spacing, and the fonts themselves are all adjustable. That's been true for the last three models of the Kobo - since the Touch came out, but they've added the ability to adjust the sharpness and the weight of the fonts as well.
Not only that, but they've improved the onscreen keyboard as well. Now the row of numbers is on the same screen as the letter keys, making it easier to use.
The Aura HD has another feature that puts it above the rest of the Kobo E-readers as well: the storage space. My understanding is that this e-reader has the most storage space on board of any in its family, at 4 GB of storage, and a slot for a micro-SD card, so you can add even more storage if you need to.
Those are all positives. There is one negative I've seen with the Aura HD. They've removed the Free Books list from the menu. The books are all still available for you to download. They're just not listed together for easy browsing. Now you need to know exactly what you're looking for before you start. How big of an issue this is for you depends on the kind of reader you are. Personally, I've not used the free books listing with my Kobo in quite a while.
I honestly have to say that I've been quite tempted by the Aura HD.