Water proof is high on the list for me. Then you don't need to use plastic pack liner, and you can pack meat without blood dripping through.
Lightweight but robust.
I like having features for alpine climbing, as a lot of my hunting incorporates climbing. e.g. Crampon Pad and elasticised cord to hold crampons. Loops for ice axes.
Good top pocket for Map,GPS, cellphone, Binos, Torch etc.
Waist strap, Chest strap.
Good quality "Ladder" webbing on straps so that buckles don't slip, otherwise you are forever adjusting your pack straps.
Quite good to have a couple of Carabiner lops on the waist strap so that you can hang gear like gloves, while you're taking a shot.
Good shoulder movement, some packs are too tight round the chest and limit ability to raise your rifle with pack still on.
Top of pack below head, to make easy going in thick scrub.
Pack narrower than width of back, again to make easy going through scrub.
Sound proof material (e.g. Fleece/stealth finish) so that you don't get scraping noises while stalking.

SOUTHERN MAN.
"GO HARD OR STAY HOME"