My perspective on "How light is right?" is a little different. I originally got into lightweight camping because I was looking to recapture the simplicity of my early trips - when I was broke, had to outfit both my Scout-age son and myself quickly, and couldn't afford a lot of bells and whistles. That make for a base load of over 20 pounds - inexpensive gear (a la Eureka/Camp Trails/Coleman/K- or Wal-mart, etc.) is heavy gear. But, we didn't have much gear, so we didn't really think about it - and we had a ball.
As the years unfolded, and my passion for the sport grew, I replaced everything with better gear - lots of bells and whistles, and lots of extras, and eventually a Dana Terraplane to haul my 35 pound base load around in. Comfortable? You bet - but I needed to rest well after lugging it all around on those high-mileage (10 - 12) days.
Over time, I noticed that I was focusing more and more on gear: setting things up, taking them down, packing, sorting, unpacking, drying, etc. - and I no longer had that "footloose" feeling from the earlier hikes. So, I started simplifying. Leaving out the stuff I didn't truly need (the second pot, complete change of clothes for a weekend, etc.) was a big help, and obviously lightened the load. But the 25 pounds of stuff that was left still required a lot more attention than I wished.
So, I started replacing things, placing a high priority on bells and whistles that worked. My Whisperlite stove (plug it in to bottle, unplug it because I forgot to put the heat shield on, put the heat shield on, plug it into bottle, assemble windscreen, pump, prime, light) got replaced by a Pocket Rocket (screw canister to stove, twist valve, light); my two man Moss mountain tent got replaced by a series of tents, culminating in a one-man Hubba - one pole, 6 stakes, and the fly-only pitch gives me a quick-pitch lunch shelter in the rain. I quit cooking meals that took two pots, simmering, and combining the sauce from pot 1 with the pasta from pot 2; freeze-dried meals may not be a gourmet's delight, but they're easy (the Lipton side dishes are almost as easy.)With simpler meals, I only needed one pot - and eventually ended up with a Titan kettle, which eliminated the need for a cup. You get the idea: simplicity, not weight, was the driving factor. But, a side benefit I noticed was that my pack weight went down to about 18 pounds. Hmmm.
That started me consciously trying to reduce weight, and gradually the ultralight thing took over. I actually got down to 10 pounds, once, but found that the lightest gear was adding to the fiddle factor: Aqua Mira had to be mixed, then added to water, which then had to be allowed to sit; the Silshelter and bug liner (I hike in the Midwest; bug protection is not a luxury) required two hiking poles and thirteen stakes and took about 30 minutes to get the pitch right (though "almost right" was the best I ever managed.) I carried it all in a Granite Gear Virga pack,but found that it was a hassle keeping the folded Thermarest pad positioned just right to be a frame while I packed everything else - it usually took two tries to get it all just right.
So, after going back to the simplest gear I could find, regardless of weight, I've decided my own sweet spot is at 14 pounds, with a Vapor Trail (so comfortable I forget to take it off at rest stops), Hubba tent, Miniworks water filter, and Pocket Rocket stove with Titan Kettle, plus a few other carefully-chosen items. It's just the right blend of simplicity, comfort, and weight - the footloose factor is once again high. (If you'd like the boring details about my gear, leave your email address and I'll forward the Excel spreadsheet.)
A side note about sub-5 pound loads: One thing I keep noticing is that there's a strong flavor of enduring, rather than enjoying, the trip. I don't want to disparage them; it's just that my priorities are different. I think the best thing SUL will do for me is spur mainstream gear makers to make the gear I like even lighter, which will let me lower my pack weight without raising the fiddle factor - and for their pioneering efforts, I thank the SUL'ers.