I've found that one of the most valuable features in an ebook reader app, after settings such as text size, font, and brightness, is the ability to lock the orientation. Unfortunately, this feature is often neglected on ebook reader apps — Bluefire Reader just recently added it (which made me very happy, since lately that is the app that I use the most). Stanza also offers the ability to lock the orientation. The Barnes & Noble eReader, their first app, also had this feature, but the newer Nook app does not — in my opinion, a major oversight.
The orientation lock is nice because if you accidentally turn the phone a little bit, the orientation won't shift, which is a pain in the butt because it takes a moment to get it to shift back (which interrupts your reading). I also like being able to lie on my side and read, which doesn't work very well if you have to hold the phone upright to keep the orientation from changing.
Hopefully Barnes & Noble will wise up and add that feature back to their new reader app. The goal, after all, should be to make reading an ebook as similar an experience to reading a regular book as possible — except where it's better, of course!