A beginner book of any level should grab your child's attention quickly. It should have a fast moving plot or lots of interesting facts. There should be something happening on every page.
Beginning readers also need some predictability because it makes it easier for them to work out unknown words. But there should also be some surprises to make reading interesting.
The book should have space between the words and lines. This is because if the words and lines are close together like in books for older children and adults it makes it very hard for your child to read them. Your child needs extra space to stop things from looking cluttered and to help them work out letters and words.
Commas should be avoided unless they are at the end of a line. This is because commas are advanced punctuation. They make reading more difficult. But if they are at the end of a line a child can usually still read the sentence without finding it too hard.
Beginner readers also need clues to help them work out words, at least until they are comfortable reading Level 3 books. Repeated words and clues in the pictures about the story are important in Level 1 and Level 2 reading books. Without these clues reading is hard for your child. Clues also help your child learn about context and comprehension which are two important things for being a good reader.
Finally make sure the pictures don't overwhelm the text. Words should always be on a plain background. White is best but other colors can work too as long as the words can still be read easily by your child. (Pages with dark backgrounds are the hardest to read.)
This picture shows the classic Level 1 reading book Bears in the Night. Another good example of a Level 1 book. It's available from Amazon.