Speaking for myself, the killer apps on my Kobo Aura One are the OverDrive integration and Pocket integration--the latter especially as when I'm reading long articles on my phone, I sometimes quickly save them to Pocket for the more comfortable reading experience of the KA1.
I don't really mind sideloading (and I had to as I have a large Nook library as well as books from other indie companies--pouring one out for Fictionwise) but would LOVE the ability to email books to my account and have them sync across devices. LOVE. I'm basically only posting this in case someone from Kobo reads this.
As to the original question of the post: what would it take for Kobo to make inroads into the U.S.? I'm not sure they can. I think most people want their devices to Just Work and at this point, all of the devices work well enough for most people. And Kindle has the ecosystem. The Kindle exclusives probably hurt, too...I really only bought a Kindle (Paperwhite, on sale before Christmas a year ago) for books that are exclusive to Kindle. I know I could deDRM, convert, sideload, blah de blah, but when I could buy a decent Kindle for $99, why bother? I'm sure I'm not the only one.
But if Kobo asked my advice for improving their marketing in the U.S., I'd say sell the capabilities such as library access and Pocket. And put integrated OverDrive on ALL the devices (unless they've done that already when I wasn't paying attention). Those are the two things that really separate them from Amazon IMO.