Thanks for the credit, John :)
I agree that you can get the Big 3 right and still have a lot of "junk in the trunk" and end up at the top of the hill with a load of stuff you don't use or need. You must weigh everything to get a real grip on pack weight.
As John and I both have mentioned, clothing represents at least as much volume, weight and cost as any of the Big 3, as well as comfort and safety. A good clothing system can be the hardest to understand and takes some trial and error to suit the user and the season/climate it will be used in. It means coordinating a number of items to work together efficiently, and there isn't much else in the UL gear universe that has as many components.
There are a lot of packs out there, but there are just a couple concepts to grasp: the weight and volume of the gear to be carried and the actual weight of the pack. Your gear fits and it is comfortable, or not.
Sleeping bags are warm enough and light enough or they aren't. You like quilts or you don't, and there are no rocket science concepts to grasp. You can argue over down vs. synthetic forever. Most of what you need to know can be gleaned from reviews; accurate weights and temperature ratings are much of the issue.
There are really just a few types of sleeping pads, and it comes down to user comfort and acceptability-- long or short, more or less padding, Spartan light or cushy and heavier.
Same thing with shelters: they keep you protected from the weather and are acceptably light, or not. You can handle Spartan tarps with no bug protection, or you want something more complex for the features you demand.
Cooking gear comes down to pots of appropriate size and a stove/fuel system that suits the altitude, temperature, and fuel availability. Lots of models, but all-in-all, the differences between the whole range of alcohol stoves is fairly small, and much the same with propane canister rigs. Take all the acceptable models and you won't have much more than a 25-30% difference. For a canister stove we're talking a couple ounces at most. The difference between a 600ml titanium pot and a 900ml one is really quite small; you need the volume or you don't.
Throw clothing in and you have seasonal variations, climate (hot/sun, cold/rain), and the metabolism of the user. You need shoes, socks, briefs, pants/shorts, rain protection, insulation, base layers, etc, etc. The biggest conceptual break for a lot of newbies is not having fresh underwear or socks every day-- wanting to have 3 of everything. From there it is more attachment to a particular garment that is already owned or an inefficient layering system. Windshirts and base layers can be a strange concept, with users still stuck on commuter/city-style wardrobe of button-down shirt and a mono-layer of insulated jacket, etc. At any rate, you can have something like a dozen items that need to layer and work as a system and not weigh more than 3 pounds or so.
And as many have said, there is the sneaky-weight that creeps up on you and that is where a good little scale can save you headaches. It is also a place to save a lot of money. Weight saved on extras is the cheapest area to control pack weight. UL insulation in sleeping systems and clothing is by far the most expensive item on your gear list, but shaving weight on stuff like toiletries and small consumables is comparatively cheap. I was surprised to check my spreadsheet and see what soap, bug repellent, sunscreen, ointments, and other personal hygiene items weighed in aggregate, even after decanting to smaller containers; "stuff" adds up. This is the place where people start sawing the handles off their toothbrushes. It costs nothing, where an upgrade to a truly light down sleeping bag will cost hundreds. Ad infinitum.
Finally, there is the snowball effect, where less and lighter gear with smaller volume allows smaller, lighter packs, lighter shoes, etc.
What is really important is that you know that you have control over what goes in your pack. It really comes down to what your acceptable comfort level is, and you make choices on it with every turn. The main thing is that you *choose* to carry a 20 pound load or a 50 pounder-- there is no "must." Ray Jardine points out that much of what we carry with heavier loads is based on a fear of nature and the outdoors. Once you get over that hump, you start leaving a lot of junk at home.