1. Rhyme, rhythm and repetation
2. Phonemic awareness
4. Good amount of graphics and pictres
Choose a book with a lot of sight words. The words should repeat, using mostly the same words but making new sentences out of them. The book should also be on a topic that the child likes, so he or she can read the book over and over again.
Being of age-appropriate topic is important for some students. Being able to read is a joy in itself, but for those who struggle they can be keen to have a topic choice fitting their age - I am thinking primarily boys here. This can be easily assisted with use of non-fiction readers with exciting topics, rather than some emergent reader storybooks which can seem babyish.
The visual aspect is crucial, and it helps if a book 'looks' new. Dusty ten-year old readers are not stimulating for any new recruit!
The above posts make great points. The book is going to have to "look" interesting in order to get the new readers initial attention. I would also recommend that you look at the print size and fonts of the book.
Normally I would add interest to the list of things to consider before selecting any reading text at any reading level; however, the group to which you're referring is naturally eager to read just about anything--as long as it looks good. The visual aspect of a text may not be your first priority as a teacher, but I assure you it's the first thing which will win a new reader over to a book. (And that's actually true more often than not, I'm sorry to say, for every age and type of reader.)
As #3 points out, there are going to be no hard and fast answers to this one. Instead you need to think in terms of the needs/skills/abilities of your individual learners, and depending on their learning styles you will have to make a selection accordingly. A general rule is to ensure that there are a good number of pictures to help students contextualise what they are reading, and act as an aid, but apart from that you need to consider your learners.
I think that trying to choose what types of beginning reader books to offer students "in general" is one of the toughest things a teacher might be called upon to do. Children, as they do so much of their learning, learn to read in different ways, since kids have different learning styles.
The visual learner would really respond to a book with pictures and graphics that connect to the text. They would also respond to a book with lots of repetition of words, so that they can visually recognize the word the more that they see it in print.
An aural learner would respond to the rhythm of rhyme and repetition, especially if the book is read out loud along with the teacher and/or other students.
This sort of book would also be good for a more physical learner as well. To chant and dance along with a book -- maybe with the pages of the actual words projected at the front of the room for students to refer to while moving in space, rather than sitting at a desk.
Again, however, different approaches suit different students, there is no catchall for every emergent reader.
The things a teacher should look for is how interesting the book would be to a reader. What is the likelihood that it would engage the reader? Then the syntax level of the book is next. How long are the sentences? Is the vocabulary difficult? Is the syntax quite complicated? And, how long is the book?
4.good amount of graphics and pictures