There can be thresholds based on pack comfort certainly. The straw that breaks the camel's back, so to speak. 31 pounds and my pack drags at my shoulders more, at 30 it feels pretty good.

Just picking up a pack, before and after you add a liter of water, you can feel it. (Yes, the water is 2.2 lbs. per liter.) Can't necessarily feel the difference between 21 and 20 on my back however.

I'd imagine a lot depends on your pack design/fit/comfort if you can notice one pound.