Suggestions:
1) Find a way to skip to a certain letter in an alphabetical list. I hate scrolling for (what feels like) minutes just to find one album. iTunes did it with the touchscreen, but I'm sure there's a clever way of doing it otherwise. For example, you could include a setting that makes you choose the letter you're looking for.
Example: Artists -» M -» Mumford and Sons.
That way, I only have to scroll down to M, and a few steps to Mumford and Sons, instead of scrolling through all the artists I have before Mumford and Sons; 113 in my case.
It could either be a simple list with all the letters (hiding the ones associated with no artist), or even make the letter appear on the whole screen and change as you scroll.
Another way to do it could be that when you hold a specific button down, the scrolling jumps from one letter to another (for example, you hold down the fast-foward button, and when you turn the wheel, it goes from Adele straight to Björk and from Björk to Caravan Palace, instead of going through all the other "A's" or "B's". Importance level: 5/5
2) Album artwork in the "Albums" list is great, but if it could be extended to the Artists list (by doing a slideshow of their album covers), that'd be super. I don't like the idea of having to find a picture of each band I listen to, and frankly think most people won't. Moreover, I mostly recognize artists/bands by their album covers, and not what they look like (most of them I never saw their faces, and strangers popping up in my screen don't appeal to me). It could be a setting so people could choose between the two. Importance level: 3/5
3) Aesthetically, the circle thing is ingenious, but it hides the artwork, even if it's see-through. I'd stick to the classy progress bar, or at least put it as an option. Importance level: 2/5
4) The font changes between pages, which makes the eye twitch. Please, please keep it to one font at a time, varying only in size/boldness. Importance level: 3.5/5
5) I'd suggest customizeable font/colors. I find the font too bold and large at some places, and Cyan isn't my favourite either; it makes me think of TRON. I'd really appreciate the possibility to choose between classy and extravagant styles. Importance level: 3.5/5
6) Another thing that would be super cool is if the battery wasn't a unique model like that of the HM-801's, and could be replaced more easily/affordably. I am aware that means less battery sales for you guys, but that would also probably mean more HM-901 sales. Importance level: 4.5/5
7) As previously mentioned, a quick way to switch between regular and shuffle modes would be great. Importance level: 4/5