To add to already good answers:

NOTEs:

"[Phone]" marks features that have more impact and usefulness on a smartphone and may be less important on a tablet.
I won't delve into content management/library organization features (e.g. dealing with multiple books), as that warrants its own Q&A in my opinion, and was also mentioned as a bullet in Jason Down's answer. If Jason doesn't mind, I can expand in my answer.

Readability/UI.

Night reading mode
[Phone] Optionally hideable UI elements to free up screen (menus, navigation, buttons, Android notification bar).

Good design usually shows miniature clock and battery indicator in the corner, that way you can still see those 2 important notification bar info indicators while reading without the bar taking up space.

[Phone] Support for locking the reader in a landscape/portrait mode for when you accidentally flip your phone sideways - or vise versa, ability to choose the orientation of reading independent of OS.
(already noted in another answer, but so critical I have to mention here as well): Support for customized fonts, and if possible, colors.
Disable CSS (seems like a small deal, till you meet an ebooks where the publisher evidently fell in love with Geocities style of content design :)
Support for reading books aloud (Text-to-Speech)
Ability to select text, and copy it or share it to another Android app.

Navigation:

Ability to advance through the text by tapping large areas of the screen (typical good design does page up on clicking left 1/3 of the screen, page down on right 1/3 of the screen)
Easy random jumping (go to page #N , go to table of contents in single clicks)
Full text search!
Book progress indicator, ideally with 2 features: clicking on it in the middle lets you actually advance into the book to where you pointed; and being able to hide.
Bookmarks!
[Phone] Ability to page up/down by using volume up/down hardware keys. Critical for single-handed reading on a phone.
Autoscroll if you like that feature (personally, I never use it)

Supported content - probably most important!:

Support for varying e-book formats.

Your main reader app should at the very least support the major ones (ideally, most of: txt, html, epub, prc/Mobi, pdf).

Support for your main e-book format (e.g. if 90% of your ebooks are in Amazon Kindle format, it's important
Support for DRM on books you freqently use may be important for some.

If it's important for you, support for your preferred ebook vendor (Google Play Books; Kindle/Amazon book store, Apple, Samsung).
If you read ebooks in other languages, support for popular encodings for them (nothing more annoying than trying to read a Russian ebook and finding the app doesn't support KOI-8).
Support for network- or cloud-based libraries
Related to the last one: ability to sync up the book positions/bookmarks between different devices running the same app (I know FBReader does that via a helper app, not sure if Kindle reader or Google Books does).
Ability to share a book via Android share menu, from within the reader.
Ability to manage book collections/libraries, if that's important to you

Reality check features

Price (and if the app has a free/paid version, which features are important and in the non-free version). For example, Calibre for Android is a paid app (Windows one is free)
Last but not the least speed of loading and speed of reading. On my old phone, Kindle App was absolutely horrendously slow, to the point of being unusable.

Less popular but occasionally cool:

Amazon X-Ray
On the fly translation. I know Google Books and FBReader support it, but seems more of a gimmic, IMHO.

If you don't plan on editing your eBooks on a PC, you may want to consider editing abilities of the app

Editing metadata
Editing content.

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