A great daypack for hunting will keep essential items such as water, layers, snacks, map, compass, first aid, optics, and gear for repair handy without becoming a distraction. It also will help make better field decisions when necessary by alleviating fatigue which can lead to poorer judgment and Cutting corners when things begin to deteriorate. Compare packs on the basis of how well they fit, how much they can hold, how you will access gear in them, and how they will be constructed to best serve your needs rather than simply comparing the weight of lightweight models to heavy laden models.
Start with the load you will really carry
Make a list of all the gear you intend to carry on a daily basis and then compare this to the gear that your guide will be supplying, that which will be shared with other members of your party, and that which you can leave behind in case you head out early and return to camp late. Water, food, clothing for changing weather, optics, first aid supplies, map and compass, personal medications, and a repair kit with recommendations for field repair of major gear problems for example can take up a lot of room. So make a list and then check with your guide as to what you will be needing to carry on a daily basis.
As to daypack size, choose one with a little excess room for items such as a change of clothing for foul weather, but not so much that it encourages you to bring along a bunch of superfluous items. Also, the pack must not be so small that you are forced to stick layers in the exterior pockets, or to store your water in less than ideal locations within the pack's main compartment.
A compact back pack is a good choice for supported day hunts where all camp gear will be left behind. However, for unsettled weather, long days or remote access, a more structured back pack of greater capacity will serve the hunter better. Verify with your guide what each participant will be expected to carry and select a back pack to meet those requirements.
Treat fit as a field decision, not a checkout detail
A comfort-fitting hunting backpack should not restrict movement while hunting. The shoulder straps should sit against the back of your neck, not cut into it. The hip belt should transfer the load to your hips, not dangle awkwardly. A sternum strap can be quite useful in stabilizing the shoulder straps but only if set up to not restrict breathing or arm movement.
For optimal hunting pack fit, all torso lengths and layers of clothing can be worn while trying on the pack. Furthermore, all heavy daily items (e.g. water, heavy clothing, first aid kit) should be added to the pack and the wearer should then go for a short walk, bend down, and try to reach up over their head. The pack should not feel any pressure points after a few minutes of wear.
Tingling in your hands or a hot spot on your lower back within a few minutes are all indicators that you need to make some changes to your pack's fit. It's best to make changes to a pack in the order that the manufacturer recommends, and find out if the torso can be adjusted to try and get a better fit. But in the end, if a pack doesn't stay in place on your back when it's loaded up with all of your gear, you're better off looking for a different pack.
Compare access, organization, and weather protection
Simple organization will generally outperform much complex structure and allow for the easy access of the various necessities of hunting. All day hunting gear can best be carried in a single main compartment of the daypack and then distributed to various locations as needed throughout the day. External pockets can be very detrimental as they tend to collect debris (especially brush) and can add considerable weight to the pack. External located items can also become exposed in harsh weather.
When will you need to access certain items? Will you be able to put on a rain layer while wearing gloves? Will your optics be protected from hard objects in the same place that you store them? Is there a safe place to store your phone and map? Are the individual pockets accessible by feel? A pack that opens up wide may be terrible to carry in the back country and have things spill out of it while laying on the ground.
Note that water resistance is not the same as waterproofness. There are a variety of ways to protect your water and other wet items from the elements. Look into pack covers, dry sacks, and internal liners. They each have their pros and cons to weight, space, noise, and organization, so look to see which will work best for your anticipated use in the field. Also, do not confuse this with the gear you will be required to bring for your firearms, ammunition, food, etc. that will vary from location to location and from land manager to land manager, so check with the outfitter and current hunting regulations prior to your departure.
Balance weight, durability, and quiet movement
Just because a pack is light does not mean it is the best pack for high-country hunting. Even though a thin wall material may make a pack light, there is a risk of failure of straps or zippers in the field. A pack needs to be able to handle the worst of circumstances and still get you home.
Pay close attention to the points of greatest stress: the stitching, the attachment points for the hip belt, the open edges of zippers, the attachment points for compression straps, and the base of the pack. Test all buckles and zippers while wearing gloves. Listen for any rubbing or foreboding creaks. Determine if any long straps can be secured out of the way.
Hunting pack weight does matter when comparing day packs for high country hunting but weight distribution is even more important. Within the packed load, dense items must not form a bulge against your back and loose items must not swing about when you change direction on steep terrain. Testing a pack at home with representative gear can make these important points quite clear.
Test the complete system before the trip
In preparing your hunting gear for the season take your biggest water container, your biggest pair of boots, all of your hunting layers, all of your optics, and all of your small accessories such as first aid kits, etc. and take a loaded walk around your house or yard. Pay attention to any pressure points on your body and make any necessary adjustments. If you have a guide for your hunting trip, add an equivalent amount of space to the pack for the guide's shared gear.
Write down all of the potential problems with your new pack (strap slippage, lack of access to essential gear, excessive noise, lack of rain protection, pressure points on the back of the hips and on your shoulders, and room for a removed layer). Then one by one solve each problem. (It is almost always better to buy a different item rather than adding on after purchase a lot of after-market gear that can in itself cause a host of new problems.)
Mistakes with selecting a pack for hunting include looking only at the capacity of the pack, copying another hunter's setup and forgetting for extra layers, and using a school or travel pack for hunting on steep terrain. Also, do not overpack with extras for possible emergency situations and forget the minimum required gear as listed by the guide and local hunting regulations for the specific area for which you are hunting. A good pack should enable you to hunt in a conservative manner, within your physical abilities, in good weather, and in compliance with all hunting laws and regulations.
Lastly, make sure the pack fits properly after you have purchased it and with the clothing you intend to wear. Check the hunting area's current rules regarding gear as well. Ideally, the pack should be unnoticed once you are on the hunt. It should be stable, easy to access, and quiet. Ideally, it should not even be noticed once you are out in the field.