Like Daryl, I prefer a pack around 2800-3000ci (45L-50L.)
This is about where we start differing, though.
It should be frameless for UL packing, but also have the ability for stiffening of some sort for long trips with a lot of food, water and fuel. Since, I have been out for up to two weeks with less than 30lb (and more than two thirds of that in food for the 14 days,) It should carry comfortably at 30lb. Or on UL trips with 11 pounds including food, fuel and water. Sleeves for your pad will help, also, maybe a couple arrow shafts. GG's older Miniposa used to build them into the pack, although removable for short trips.
Side pockets are a must. one for the stove/fuel/kitchen stuff, one on the other side for water bottles (2-gatoraid bottles.) Over the course of a 20mi day, the balance will change a bit, but not too bad if the center and front pouch are well loaded. I notice it more with light loads than 20lb loads. The compass and map go in my pocket or are around my neck. There is really no need for a third pouch, though it comes in handy to collect trail garbage.
A "hydration" sleeve is nice for flat stuff: spoon, tarp stakes, aluminum foil lids, etc... I never use a bladder, though. After all the pipes & valves, they are no lighter, and they require more fussing. Anyway, over the years, I have decided that I actually like the sleeves, though not for the intended purpose.
I prefer an external pad sleeve, like GG packs have, letting the pad do double duty as a frame for the pack.
A mesh front pouch for holding rain gear, a removable fleece jacket and tarp pretty much rounds out the pack, itself.
Hip belt pockets are handy, but, not necessary. Again, a couple bars can be put in a pocket for the day's hike. They are also handy for a camera, should I bring one.
Shoulder pouches and bottle holders don't work for me. I have problems with a vertibra/disks in my lower neck and anything that puts a lot of strain on my shoulders can be painfull, especially at night. I made a couple smaller ones but quickly found they didn't work for me.
As you can see, weight transfer to my hips, and, weight off my shoulders are important. Soo, a GOOD hip belt is needed. ~5-9lb is about all I can do for any length of time on my shoulders. Short periods of a lot more weight are OK, though. So, padding becomes important with a full pack. A wider belt helps too.
Compression is not that important, but makes it easy with small loads. A couple loops of elastic is all it takes, though one of my packs uses straps...it works, though because it is a solid pull it bent the Caldera cone, one time.
The weight of a pack is quite important. It needs to be less than 1 pound. I think the older Miniposa said 14oz, but mine currently weighs 16-1/4oz with two additional belt pouches and several patches. The G5 was patched to beat hell, but was retired this season after it slid down some rocks (me with it. Hmmm...they call them slides for a reason, right?)
Anyway, I am starting to keep track again, looking for a sub 8oz pack with all of the above... Oh yeah... It *must* fit. And be a bit more durable than the venerable G5-Spinnaker I used for so many years. Oh, yeah...it must fit a bear ball...
Reading carefully . . .