If you are looking for a solution that allows you to walk for 12 hours, or for multi-day hikes, then I suggest that you need a proper walking backpack, which you can adjust to carrying your camera gear, rather than a dedicated camera backpack. Packs that are designed primarily for long distance haulage (sometimes called technical packs) will almost certainly be much more comfortable than a camera backpack once you are carrying any kind of weight, or for a long time.
That being said, I just read a review on Luminous Landscape (http://www.luminous-landscape.com …backpack_the_rpt_p3.shtml
) of a pack that I have never heard of, that seems to be a camera backpack with a technical harness - you should check it out.
My personal solution for multi day treks is to use a technical pack, with a small toploader bag (carrying body plus one lens) mounted on my chest for quick access. No easy access to extra lenses, but a good all around solution (for me).
In answer to your questions:
1. If you are carrying any significant weight (different for everyone, but for me, say over about 7-8 kg), then a proper waist strap and sternum strap will certainly make a difference. The waist strap is more important. A good waist strap (by good, I mean padded and fitting, preferably independently slung - not just webbing attached to the bottom corners) will shift most of the weight to your hips, and off your shoulders. In my experience, any bag that has a proper waist strap will also have a sternum strap.
2. Side access is handy, but introduces compromises. I have a Kata 3n1 20, which I find quite handy (and comfortable in the cross strap set up). On that bag, the side access means that you have less versatility in outside stowage (since the side access needs to be clear). It also has minimal extra storage (e.g. no water bottle mesh pocket).
3. Balance of tripod carrying position depends upon the weight of your tripod and distribution of weight inside your pack (i.e. you should be able to adjust your gear to balance the weight of a side mounted tripod, if necessary). In my experience, with a proper harness, I have never noticed any balance issues from having any weight slung on the sides of the pack. Centre is preferable.
Personally, I don't like having anything hanging off the bottom of my pack. It may be a problem if it sticks out sideways (like a tripod might), and you will be putting the pack down on it all the time. In addition, for load bearing efficiency, it is better to have weight higher rather than lower - you could strap it to the top of your pack (but you're more likely to bang your head on it!). If your pack is big enough, small tripods can go inside the pack, as an option.


