Firstly, happy thanksgiving!
Ok, a perceptive commenter asked what I look for in a good textbook. Here are a few things:
(1) SIMPLE – scholars rarely really understand just how little students know. Keep it simple. If you give advanced information, do so in an appendix or in footnotes.
(2) CLEAR – going along with #1, make sure the textbook progresses in a clear way, from paragraph to paragraph and chapter to chapter. I am using a textbook next quarter which is not as clear as I would like, though it is as simple as I would like.
(3) Graphics – charts, maps, diagrams, and pictures really do make a textbook more appealling for me.
(4) End of chapter quick summary – to help with clarity and simplicity!
(5) For Biblical Studies, textbooks should engage with the “so what” question when it comes to what information to include. Too many texts offer random facts and background information without really dealing with WHAT THE TEXT (e.g. NT, or OT) is REALLY ABOUT. Perhaps this is a conscious ideological choice. So be it. I am looking for more. It can be general or exploratory, but it needs to be more than just “facts”.
(6) Good bibliographies at end of chapters – best if it is annotated.
(7) Avoiding ideological extremes (e.g., extreme either in naive orthodoxy [=apologetics] or pure skepticism [=hermeneutic of suspicion])
I am sure there are other things. For me, cover design is attractive, but I try not to judge… 
Nijay, thanks for this follow-up post! It is very helpful.