First, thoughts on an XUL definition:
While it's tempting to follow the current trend of using the previous base pack weight tier divided by two--i.e., Lightweight is considered to be sub 20 BPW, UL is generally considered to be sub 10 BPW, SUL is considered to be sub 5 BPW, which would logically make XUL a sub 2.5 BPW--once you're hitting this level (really, once you're even at SUL) the clothing you wear and poles you (might, or in my opinion ought to) carry are too substantial a portion of your load to give any measure that doesn't include them salience. It is therefore necessary for XUL to be FSO based.
At these weights, many Sub 5 FSOers won't be "backpackers", as the activity is understood from a traditional perspective (or lightweight, ultralight, or superultralight) but adventure racers or ultramarathoners and the like. Unfortunately, for these (awesome and crazy) pursuits a 5lb FSO may be considered heavy!
XUL then, need specifically refer to the weight for a backpacking loadout, including "the big 3" of sleep, shelter, and cook systems. I feel without these, the pursuit is no longer properly "backpacking", in which case the weights to which XUL is supposedly relative no longer exist, and the definition thus void.
Moreover, the loadout should be effective for 3 season trips of several days on Class 1 and some 2 terrain in a variety of weather conditions; that is, temperatures down to freezing with the possibility of sustained moderate precipitation and considerable winds with several miles between water availability.
As a big guy, I wish I could create a rigorous definition that allowed a higher weight based on height or mass or foot size (even light size 15EEEE shoes weigh perhaps half a lb more than average sized shoes of the same model, so there goes 10% of my total loadout right there!), but I can't think of one that wouldn't be arbitrary, and since 5lb FSO is arbitrary already (like LW UL and SUL are as well) I think it's better to be simple and arbitrary than complicated and arbitrary.
Definitions set, we come to the point:
XUL thus rigorously, if arbitrarily, defined as 5lb FSO, the best goal, in my opinion, of an XUL gearlist (or any gear list for a category where weight is the given) is not to get simply lighter than the defined weight, because the weight is given, but to hit that weight more elegantly: more comfortable, practical, cheaper, etc.
Alan Dixon (Dixon=Adventure?) perfectly demonstrates this idea of elegance in load with his "full comfort UL gear list" (http://adventurealan.com/lw_gear_list.htm). He's not trying to be lighter than the arbitrary UL definition, he's trying to do UL as elegantly as possible. Likewise, he included a cook system in his XUL Appalachian gearlist(adventurealan.com/2-4_index.htm), because he felt it more elegant.
People will say the above definition is arbitrary and ask what purpose it serves in practice. I agree it's arbitrary, as the definitions of LW and UL and SUL are arbitrary, and the purpose this lower weight tier serves is exactly the same as those above it: longer miles, more comfortable hiking, etc. People will then ask, "does XUL make a practical difference over SUL or even UL towards the achievement of these goals?". In fairness, I don't know, I believe it will make SOME difference, but I don't know. What I do know, is that it sounds like a fun exercise to me, and I think it's an exercise that will inspire and push our little pursuit and its supporting industry forwards.
Earlier in this thread, Craig and I agreed to assemble and test XUL loads in the near future. To Craig, I invite your support of the definition I've laid out above by chasing the greatest elegance possible in our XUL loads, rather than making it a challenge of simply going lighter (I think at 5lb FSO that challenge is highly present!). I think this will make for some fun and standardized pseudo-competition, and since "elegance" is so subjective, it will make for some great debate about the various and necessarily creative solutions to the XUL challenge.
Credit where credit is due: Much of the above definition is inspired by Adventure Alan's article at http://adventurealan.com/2-4_index.htm. In fact, his loadout will be my starting point in putting together my own XUL list for this challenge. I hope I've contributed something to the logic behind the definition he practiced. AdventureAlan.com was my greatest inspiration when I decided to dabble in UL, and continues to be where I direct all newcomers to the pursuit.
Finally, on a personal note: I've just moved to the SF Bay Area and know no backpackers here of any sort. I'd love to meet and hike with anybody in the area! Having spent my whole life reflecting on and emulating that which I admire, I am as awesome as my talents have allowed me to embody the word as I have defined it. In arrogant summary: I am awesome*, let's meet up.