I guess it all boils down to how someone uses their device.
My reader is my TBR pile and I usually have 50 to 100 books on it. As soon as I'm done with a book, I remove it from the device. So a few simple collections are all I need. But I can see how a more sophisticated collection or folder management system would be important for someone who keeps more books on their device.
As for me, I typically read across three devices: my reader, my iPod Touch, and my Android phone. So for the way I use my device, the wi-fi, syncing across devices, and search function are the most important features.