Here's my take (generally and specifically):
1. I think that camera carrying gear (be it a backpack, sling, or holster) really is a very individual thing. What rides well on me may be painful for you. Additionally, it varies with the nature of shooting plans and gear: are you just schlepping stuff from A to B and then you shoot at B...or are you doing street photography and pulling lens and gear out continuously. It depends a lot on the nature of the shoot (I like to use my Tamrac holster...except when I'm using my 200mm f2.8 zoom which can't handle that so I use something else like a messenger bag instead. So I really don't believe in "one size fits all" when it comes to camera equipment carrying devices.
2. I think there are a couple of big variables with any backpack:
--do you expect it to carry stuff other than camera gear?
--do you design it so it can work as a quasi-sling (I've done this with a bunch of day packs back in the day...would wear it over one shoulder only and then have the side on that shoulder unzipped so I could access stuff without pulling it out).
--how secure do you want it to be. I love backpacks with lots of external pockets (so I can access a lens or cleaning equipment or filters or speed light without having to take the backpack off and unzip the whole darn thing). But having those external pockets really isn't very secure when you're in crowds with pickpockets (unless you wear the backpack in front of you).
3. Here are the features that I personally like on a functional backpack:
--good protection of my gear from water and enough padding on the bottom so if it's dropped my lens have a chance of surviving.
--lots of external pockets.
--lots of internal pockets. It may be primarily a camera backpack but I'll put a small notebook and pen and releases in it, batteries, filters, SD cards, maybe a cellphone with headphones and sunglasses, maybe a water bottle and trail mix, small flashlight, maybe duct tape and a weatherman, maybe a couple of flash drives and a speed light. Plus AA batteries for the speed light and sometimes an emergency space blanket. So probably 2/3rds of the total items I put in there are not going to be big, they won't be bodies or lens. So I need small pockets so they don't rattle around or fall out if the pack gets turned upside down.
--a handle on top (so I can carry it like a bag when I don't want to have it strapped on to my back or slung over a shoulder).
--versatile accommodations for tripods (so we're probably talking velcro loops on one side).
--I can't tell you the number of times I've used a backpack or sling as a sandbag for a portable soft box on a stand or a reflector. So enough loops and velcro to make it easy to convert that backpack into a weight/sandbag when I need it.
--I like camera gear carriers that are not obviously camera gear carriers (i.e.: backpacks and slings). On long flights overseas or when I was waiting, I've removed thread for Nikon or other obvious makers on the bag. I figure as a westerner, I'm enough of a target, I prefer not to become even more of one by saying Canon or Kata or Lowepro on my stuff. So if you must do a logo (as you must), I prefer it smaller. And I prefer colors in black or dark with at best maybe a small strip to make the bag stand out from 30 other black bags.