I gotta have zippers and also someone is missing a bet by not making a pocket in the top that is shaped, zippered and easily accessible.
I do like the Owyhee holdback system as it keep the panniers from hitting the goat in the shoulders going down hill.
I also like the straps that go across the top to pull the sides up and the bottom out.
We'll use the pockets to make a quick weight adjustment, that's about it. With a larger pack string, all those pockets seem to eat things that end up not being found until the trip is over. 12 goats, 24 panniers, 48 pockets, now which pocket did I put my missing stuff in?????? We try not to use them.
Durability is top on the list, a failed pannier has no place in the back country. I like the flap closure with a drawstring top on the compartment.
I have to agree with Herb that the pockets are just a good place to lose things. I would rather have different colored bags that I can put things in, and then put those into the panniers. I also prefer a system with a minimum of zippers, such as a flap closure with a drawstring top. But, until they fail, the zippers are very nice to keep goats, chipmunks, mice, spiders, etc out of the panniers.
What I've found that works for me is to use the panniers for the stuff I won't need until we get to camp (bedding, shelter, food, kitchen gear, clothes, etc). The stuff I may need along the way like drinking water, lunch, rain jackets, and 1st aid kit, I like to put in a pair of canvas tote bags (like LL Bean's Boat and Tote bags) and hang them on the packsaddle of one goat. Those are easy to access, but not as secure and weather proof as the real panniers. That goat has a lighter load than the others, but he may carry those tote bags with everybody's lunch, jackets and drinking water on day hikes, or may carry a heavier load on the way out. It all evens out in the end.