I would look at a light load as under 10 pounds, more of a day hiking load for me, or perhaps a short trip SUL load. Getting upwards of 24 pounds is getting up there and definitely more comfortable with a frame.
I switched to using a framed 30 liter pack for much of my summer day hiking last year and found it much more comfortable. There was no agonizing over adding another liter of water or taking more camera gear, I just dumped in what I wanted for that trip.
For me, the real advantage to framed packs is the ease of loading. It takes careful loading to get a frameless pack to transfer weight to a hip belt well. With a frameless pack, there is always the juggle of rolling or folding pads and arrangement of lumps inside the pack and getting a good tight stuff to create a stable column to get the weight transfer. It's fussy and adds to the hassle of getting things in and out of the pack during the day. Adding something like a bear can be a major pain.
The framed packs with a plastic frame sheet and aluminum stays like the REI Flash 65 or the modified external frame like the Osprey Exos series gives good weight transfer and isolate the load from your back, sob you can load for balance and trail convenience without concern for a skin tight stuff or items poking you in the back.
Hoop frame packs like the Gossamer Gear Mariposa split the difference, giving light weight and good weight transfer, but needing more care in loading.
The the question posed by the OP, light loads in a framed pack don't feel as heavy to me. I think that comes from good shoulder strap and hip belt padding, and the zinger being that the pack is more stable: I think part of the impression of weight is how well the load moves with you, besides the pressure points on shoulders and hips. The weight still catches up with lungs, heart and knees farther up the switchbacks, but better for shoulders and back.
I've had some back issues over the years and would look for a pack with excellent weight transfer if I had disc issues. I've used the Exos 46 with good results. I think that an UL pack with a plastic frame sheet and aluminum stays can be as effective for weight transfer but can be hotter than the trampoline back panel designs.
I think the hoop frame packs are a good compromise when you get into say 65 liters or more where the pack weight starts to add up: you are carrying enough stuff that you really have to get every item to the lightest versions, including the pack. I assume that someone using a higher volume pack is hauling more food or winter insulation, or in my case, adding more hammock gear that needed more space.
Which pack you choose from there is more like choosing a shoe, depending on how it fits your particular body type. Hip belt stability is key--- you want it to stay put without pinching or sliding down. The rest of the pack design should be pointing the weight to the connection with the hip belt. You end up with the shoulder straps being there for stability rather than load bearing.
Getting to a store with a lot of packs, some test weights and a couple hours time will tell you a lot about what works for you. You have to try them with weight and move around with them: adding 20 or so pounds really settles the pack into your body and makes it fit much differently than when empty, as well as the stability when you start moving.
Happy hunting :)