Love this question, I could talk dog packs all day! We've probably owned four or five different styles between three dogs, and honestly liked every one of them, because fit is the most important factor. If your pup is like mine, they love to lay down in puddles! Sadly, wet straps make for rubbing and potential chaffing if the fit is no good, which do not make for a pleasant hike for your doggo. We have a great overview article about Hiking or Backpacking with Your Dog in our REI Co-op Expert Advice, which talks about fit and that is really what makes a great dog pack.
Otherwise, I always look first at capacity (usually measured in liters in the specs). A lot of our dogs' backpacking gear is big and fluffy (think bed and jacket) and it is always nice to not feel like you're zipping an overstuffed suitcase! For the same reason, external attachment points or daisy chains, make it easy to clip on wet dog bowls to dry as you hike or even a second sleeping pad for winter backpacking. There are lots of other features out there, but I would give up any to make my puppy comfy in a well-fitting pack. Hope to see you and your pup out on the trails!