I'm going to say that fit is one of the most important elements.
I highly suggest you find yourself a store and try on a lot of backpacks.
Make sure you properly adjust them.
Usually, someone in a store will be able to help you out.
Additionally, the various features you listed are useful for different activities.
When buying a pack, consider what you'll be using it for and how long.
For instance, my mountaineering pack has loops for my ice axe, a special pouch for my crampons, etc.
If I were planning on a day hike, I would bring my small, 11L pack.
If I'm going for a few days, I might go for a 30L pack that has a storage space for my sleeping bag.
Also, you may wish to load the pack while it's in the store with the weight
you estimate you'll be carrying.
That way, you'll have an idea of how it will feel on your shoulders.
Definitely walk around the store at that point.
One of the issues with most purchases is that you won't really know if the pack does exactly what you want until you've used it a good deal :) Live and learn!
Identify what you do and don't like and eventually apply that to your next purchase.
outdoors.stackexchange.com/users/48/A.A. Grapsas
Pack weight - A heavy pack is also some extra Kg on your back.
Even though a good pack will put them to good use in distributing the content's weight over your back.
Pack frame -
Many ultra-light backpacks come with a minimal frame design, or no frame at all.
You'd need to use your mattress to create a simple frame. Accessibility-
The number of different access points to the equipment you put inside.
Some come with a zipper on the bottom straight to the sleeping bag compartment, some have only top draw-strings.
outdoors.stackexchange.com/users/32/Noam Gal
What really gets to me, is strap placement.
Since I'm a big guy, if the straps on the top half of pack are close, it gets really uncomfortable real fast.
So I'd also like to add strap placement on the watch-out list.
So I definitely agree that you should buy a backpack hands on.
Internet is too much of a gamble for me.
outdoors.stackexchange.com/users/40/Janis Peisenieks
Pack-Buying Priorities: Comfort Comfort Comfort Workmanship quality
Everything else
Best Materials: Dyneema (which is the same thing as Spectra)
:
Ultra high-end fiber stronger than Kevlar.
Packs made of full Spectra/Dyneema cost $1000-2000.
BEWARE there are lots of cheaper packs that claim Spectra or Dyneema, but which only use these high-end fibers for "rip-stop"/"grid-stop", with cheaper fabric in-between.
Cuben: Ultra-lightweight, durable, and waterproof.
Also very expensive.
Silnylon (Silicone Impregnated Nylon):
Lightweight but can be ripped.
Best Manufacturers: McHale Zpacks Cilogear Zen Master's Advice
: Do yourself a huge favor and go the "ultralight" route and buy a frameless backpack.
outdoors.stackexchange.com/users/155/themirror
As already stressed by others, the backpack has to be comfortable.
If the back system does not fit you, other features won’t make a difference.
But there are plenty of good back systems and therefore good backpacks to choose from, so here are a few more tips:
I like pockets on the hip belt.
Good for a small camera, change or handkerchief to keep at hand.
I like when you can adjust the height of the lid, shift it on straps.
Even if you can’t fit your things into the backpack, you can cram almost anything under a height-adjustable lid :)
If you have some special gear like snowshoes, walking sticks or ice pick, take it with you when choosing a backpack and see how you can attach it to the backpack.
Think about hydration.
If you use a hydration bag, does the backpack support it?
If you are used to PET bottles or a thermos, where can you place these in the bag?
Will they be at hand?
I’ve seen some nice side-pocket designs where the pocket is slightly skewed to the front side and can be accessed even without getting the backpack off your shoulders.
Separate pocket for the raincover is a nice touch, especially when you put the wet or muddy raincover back inside.
If you’re buying a backpack with two separate compartments, does your sleeping bag fit the lower one?
This can be an issue with the synthetic sleeping bags that tend to be quite bulky.
And of course, the backpack will take a lot of beating so it has to stand that.
If the sewing doesn’t look reliable or the zippers look flimsy, I’d think twice about buying.
outdoors.stackexchange.com/users/103/zoul
One thing I don't see on your list is fit.
REI recommends finding your torso length and hip size, and that will help you to find the right fit.
Source: REI
outdoors.stackexchange.com/users/2/ashansky
I know one can compare backpacks by the following:   Content (in liters)  Number of external pockets (for quick accessable water bottles for example)  Hip belt available or not  Waterproof / Water resistant / not   But I'm probably missing a lot of other things to which I should pay attention when buying a backpack. Any advise would be helpful.
When buying a backpack, what should I pay attention to?
outdoors.stackexchange.com
<backpacks>
outdoors.stackexchange.com/questions/17
outdoors.stackexchange.com/users/46/Jasper
Fit, comfort and strap placement are the most important elements. Along with workmanship and quality, consider what  you will use it for and for how long. Comfort, quality of workmanship, fit and strap placement are all important. You should also consider what you'll be using it for.
Fit and strap placement are key. Comfort, quality of workmanship and intended use are key considerations.