Interesting question, about what I would want. I had to stop and think - do I want it primarily as an e-reader or as more of an all-purpose device? Realizing of course that the more features I added, the higher the cost would be. I decided I wanted the device's focus to be on reading and associated research, not a broad media device. I've got my laptop for that, not to mention a smart phone.
I think that I would prefer the device to be affordable, so I skipped the items like touch screen and video player. I was going to skip color screen too, but then I thought, hmm 9+" screen is good for viewing "coffee table" books and art books, and it would be nice to see those images in color. Also, charts in color would be nice. But I can live with grey-scale, as long as there are a lot of levels.
Wifi capability and a browser would be useful - to look up things on Wiki or to download books directly. 3G - meh; I wouldn't use it. Ditto for email - not necessary.
Touch screen - no. I'd rather have well-placed buttons so that I can hold the reader and turn the pages all with one hand. Also, at least two methods of turning pages for ease of right-handed and left-handed use. I don't need to have another screen to clean and touch screens seem to smudge easily. Besides, unless a good touch screen is installed, they tend to reduce the sharpness of the image.
People seem to like having a dictionary. I'd like a music player just so audiobooks could be added to the reader. As others have mentioned, text-to-speech capability would be nice.
I looked up what OPDS means and that looks useful.
Support for a wide variety of formats is a must. As is a good PDF reader, one with reflow capability. Meta data access would be nice.
I would also like a digital clock in one of the corners. I'm often reading while waiting for one thing or another, and I'd like to be able to check the time.
Ease of creating folders and subfolders is essential, as is quick access to the files.
SD card capability is a must. The device should charge while connected to both the computer and to a wall outlet.
Provide all the essential doodads with the initial purchase:
USB cable, AC adapter, earbuds, carrying case.
Devote some thought to the ergonomic aspects of the device: make it easy to hold and operate with either hand, make the controls responsive, allow some space for clipping on a reading light. Frame colors should be unobtrusive. Give the back a bit of texture for easy gripping. Minimize the weight of the reader as much as possible, but build it strong; remember that devices like an ereader fascinate little kids and they want to hold them; they also tend to drop them often.
Allow for great flexibility in zoom rates; that is an essential feature for many of us. Also, allow more font choices than just a serif one and a sans-serif one; we should be able to load our own choice of fonts. Include an auto page turn feature. Allow landscape and portrait modes. Have a precise battery meter. An encryption capability would be useful in some circumstances.