As you say it isn't designed for power users so I won't talk about it as if it were but from a non-power user perspective the only thing I noticed is that the registration page could be a little better.
On the web-site you group the information in relevant ways either using whitespace or just descriptive text. In the "app" version it just looks like a giant block of text boxes with big purple "missing information" explanation boxes.
I'd also consider reducing the size of the window a little tiny bit, you have tons of whitespace to the left of the image running down the page which seems to have no function (you never put "content" there). It is also a much larger "border" than on the right hand side which makes the whole page look lopsided.
In general I think the application UI looks extremely professional. No clue what framework it uses (if any) or you've just spent ages making it feel like an interactive web-page with things like back buttons. Either way it works for it, and the professional grade artwork is the icing on the cake.
What is the backup service's USP?