Most of all, it needs to be quiet and rugged, not too big, and the pocket arrangement needs to feel intuitive to me so I'm not looking for things all the time. I do like a couple of outside stretchy pockets for things I grab all the time or a bottle of water, and I like a way to carry an extra jacket. For me, the Badlands Hyper-Hydro and Reactor packs are near-perfect. Capacity about 900 cu in, keeps me from taking more stuff than I really need, doesn't get in the way of shooting if I get an opportunity, light, and comfortable.
PS because I had shoulder surgery this year, I have been carrying a fanny pack, also from Badlands. Really well made, well thought-out, holds a lot and doesn't have the problem of most other fanny packs I have tried, which is they pull down on your hips after a while and get uncomfortable. The people at Badlands are really THINKING about how their packs are going to be used. Also,I have been using Badlands packs since they appeared and once I lost a zipper pull but that's about it. Good stuff.