Well, for backpacking, as opposed to survival, I think the outline for a list I gave covers it, but certainly with some flexibility allowed because, you're absolutely right, it depends on the trip.

Still, I believe that even more than what gear you have, being ultralight has to be a matter of knowing what is not essential. I'm still learning that.

I suppose it's quite possible to carry 50lbs of "Ultralight" gear for a three night trip, but you really wouldn't be backpacking ultralight if you did.

For example, with backpacking in mind, a big knife, even if it is ultralight, is really not essential, and that's more to the idea I'm getting at.

Another example; I always carry the clothes I'm wearing, the clothes I sleep in, and a change for those I'm wearing in case I get wet. But, I know I could make due with what I'm wearing for a week, and if I got wet I could change into my sleeping clothes while I dry out my hiking clothes. Even if all my clothes were hi-tech ultralight stuff, I could still do this, but I haven't yet, and that really gets back to the point that W_D made.

That change of clothes has provided me only with the comfort of knowing I had them, but in truth, they have been entirely unnecessary for the trips I take.

If I made a point to complete a "Bare essentials" exercise I might quickly gain the level of comfort I need to bring less as a matter of practice, while still bringing everything I want and need.