Back-lighting. These should be ideal for reading in bed, but because they also need a lamp turned on I might as well stay with a book. I'd assumed that the screen would illuminate itself somehow for reading after dark.
Check into the Nook Simple Touch with GlowLight. I haven't used it myself (it was announced a week or two after I bought the Simple Touch). Reviews generally praise it. In practice it's probably similar to a back-light, and it can be turned on and off, and I believe I've read the brightness is adjustable.
Page turns - maybe these were older models, but it drove me crazy that every page turn required a blank of the screen followed by a redraw of the new page.
I have an older reader (Boox) which has the screen issue you're referring to. My Simple Touch does a black screen fill every six page turns, but it's quite fast, and may be more tolerable. (The Simple Touch has an eInk Pearl display, a Kindle or other reader with the same display probably rate of screen refresh and speed.)
Lack of touch screen I'm used to swiping my phone screen - the "dead" screen on the e-readers drove me insane - always hunting for the darned page turn button, or cursoring through (not very well designed) menus.
The Simple Touch uses a touch screen (alongside page-turn buttons). I don't know if any Kindles do. (As much as I like Amazon, I went the Nook route due to epub support, so I'm not familiar with the latest Kindle releases.)
All in all it seemed to me that each of the e-readers seemed primitive compared to my Nexus S.
I've never used a smartphone, and I've only handled a tablet long enough to set one up for someone else, so I can't compare the reading experience with them. I do find eInk easier on my eyes. (Computer monitors are better for skimming than reading for me.)
The Simple Touch does have issues where the touch screen doesn't seem to be calibrated well enough, and where tapping on the screen to bring up the dictionary doesn't do anything. It's by no means perfect.
On the vendor tie-in part, I do buy books through Barnes & Noble, but I strip the DRM off before loading them onto the Nook, which makes them essentially non-vendor books as far as the Nook is concerned. However, that's not the same as library loans. I've never looked into library loan support for the Nook, so I can't comment on that one.