I really like that idea of "inflection point." I notice the same thing: when I'm under 15 pounds, the load seems to fade into the background to the point that I'm not really aware of it. Over 18, it's not burdensome by any means, but I'm usually conscious of it.

Some of this is also a function of what shape I'm in. If it's a trip early in the year, when I haven't lost the 5 or 10 pounds I put on over the winter holidays, and haven't been out for a while, the hiking always seems difficult and the load onerous regardless of weight. If it's later in the season, with the weight gone and doing more hiking, the inflection points kick in.

I don't tend to look too critically at the weight of an individual item. I tend to look first at function, then tend to pick the lightest item that performs at an acceptable level. Saving an ounce on a toilet trowel is pointless if the trowel bends or breaks the first time you actually try to dig with it, for example.

As someone else mentioned, attitude is important: you can end up adding significant weight if you keep adding items that are individually insignificant. I still perform the old two-pile test once or twice a year: when I'm unpacking after a trip, I put the essentials (rain gear, first aid, etc.) and everything I actually used during the trip into one pile. Anything I didn't actually use goes in the second pile (aka the "what was I thinking?" pile), and doesn't go on the next trip. Luckily, this pile usually only has one or two small items these days.