I 100% agree with @TravelKate . The handle on top is critical. I have had to use the handle countless times to assist my 100lb dog in getting over slippery terrain/rocks/onto platforms, etc. 

I'm sure some other packs have something similar, but i'm obsessed with the ruffwear palisades saddlebag attachment. You can take the saddlebags off without having to remove the entire pack. It is extremely helpful after a long day of hiking, or even just taking a break, to be able to remove the weight from your pup without having to take the whole pack off. It also makes it easier to put the pack on in the morning. You can slip on the harness, then attach the pack. Then you aren't wrestling the full floppy bags over your pup's head. 

Other big thing to look for is durability. My dog frequently forgets how much wider he is with his pack on and tries to slip through spaces he is too wide for, or walks too closely to rocks/trees. His ruffwear pack is 7 years old and has been used on approximately 900miles of backpacking trips and you can't tell. The one thing that has broken on the pack is a buckle. I sent an email to ruffwear and they had 2 new buckles in the mail for me that day.