I'm in the market for a backpack for fast paced multi-day hiking (away from food up to 1 week). I've tried some of the really big backpacks (65L) and found the padding bulky. I've tried some good ones around the 45-55L mark but I'm concerned about having space. There seems to be a cut off point where the newer backpack of the lowerage literage have less padding which offers a freer feeling. What kinds of other features exist on backpacks that are the most useful as there seem to be many.
I'll echo the most prominent theme in the related question, which is that above all else comfort is the most important consideration in any backpack. The worst backpack is one that ends up causing you pain after a few hours when you have to put up with it for the entire week - so trying on as many as you can is important. If you're at all in doubt after a few hours, then it's not good - that effect will multiple out many times over a week. Padding, strap size and placement, etc. will all factor into this comfort aspect, but it's difficult to advise just based on this since it's different for everyone. Some people require large amounts of padding, for others it's not an issue, some people (me) require straps that can usually be adjusted heavily, some don't really need it, etc.
In terms of other features, they're really, as far as I'm concerned, not much of an issue, with a couple of exceptions - that being waterproofing and durability. One of the worst feelings ever is arriving at camp after a rainy day and finding all your stuff soaked, and this can happen if the material leaks, or sprouts a hole somewhere (which will then clearly cause it to leak.) Lots of extra pockets, map holders, compass points, phone holders etc. are all a bit gimmicky to me and are things I rarely use - the above aspects are much more important.
This is a question whose answer depends a lot on personal style and preferences. I prefer an ultralight style of backpacking. For a week without resupply, I would use my Gossamer gear G4, which is a 66 liter pack that weighs 0.9 lb. It's basically a silnylon sack with two arm straps, plus some netting on the outside that contributes to the stated capacity of the pack. With that pack, for a summer trip, I would have a base weight (i.e., total weight without food or water) of about 12 lb. In addition to that weight, I would have about 8 lb of food. (I eat about 1.1 lb/day while backpacking.) This adds up to 20 lb on the first day, and that's a very comfortable amount of weight to carry with this style of ultralight pack.
As a matter of style or personal preference, some people like to use a type of pack that has a sophisticated suspension system, is somewhat stiffer, and weighs a lot more. People who use this type of pack are usually not practicing a very lightweight style, so their base weight for a 7-day trip would probably be a lot higher. The level of comfort when wearing the pack comes out about the same, because although they're carrying maybe double the weight that I am, they have a better suspension and more padding.
Might help if you were more elaborate with "trekking", "fast-paced", and "away from food". You didn't even mention terrain, experience, group or individual, seasons, etc. That could be anything from a large day pack to a full loaded backpack, depending on what you really have in mind, how much experience you have, the terrain, etc. Most people carry food while backpacking (everyone should), and it's a bad practice to head out with less than you'll need, for example.
To the backpacks. No, I would never recommend someone buy a backpack based on "comfort", and I and my family have bought 16+ backpacks and done dozens of backpacking trips over the past 30 years. What you find "comfortable" in the store may turn out to injure you in the field or fall apart on you. When you first put on a pair of sandals, they'll be pretty comfortable, but you shouldn't go backpacking in them. A good pair of boots won't be nearly as comfortable in the store, and may need break-in before you actually want to wear them much, but they're a much better choice for backpacking.
A backpack first has to be fit for purpose (which is why you need to define those terms some more), second has to fit you and be fitted to you, third has to be durable/fixable enough to get through the trip, and then has to be something you like and can afford. I'd recommend nailing down what kind of trip you're really planning, after talking to people experienced in that kind of trip, then go to a good reputable shop, tell them what you're doing, and see what they recommend. If you can, go to multiple stores. Lookup what the manufacturers recommend for what you're doing. Then choose your pack. If at all possible, have it fitted to you in the store by someone who knows what they're doing.