I'm not disagreeing with this, but can you give some examples? As a passenger -- other than the windows -- there's very little difference I've noticed over the course of ~40 flights on each.

From the top of my head, Boeing 787 fuselage sections are formed as a single tube (more advanced, more expensive) while a350 sections are formed from 4 separate parts (older, cheaper technology, easier to repair though). 787 Cockpit is made from carbon fiber reinforced with titanium, while A350 cockpit is a traditional aluminium one. So yes, these things are invisible to a passenger the same way as a software architecture is invisible to a user, but to airlines and to industry as a whole these things are important.

What advantages does the single tube fuselage give?

I would imagine that without skin splices it weights less, has less drag in flight and generally one less thing to worry about for mechanics.