Quite right, I had a look at the schematics, and many of the changes (e.g., using FETs and op-amps to auto switch the Vin source) I thought came in with the Uno actually came in with the Duemilanove. The Diecemila design is actually much closer to what I was thinking of interms of "pleasing simplicity".
I'm also surprised that in this thread everyone (except lefty) has nominated a derivative rather than an actual Arduino design as their favorite (which is what I was thinking of when I asked the question.) Personally, my favorite would be the Teensy 3.1 (which I think is approaching a work of art) if I was thinking of including derivatives as well.
But what I was wondering, I suppose, is how people see the general direction of Arduino board designs as they've developed over the last few years -- Leo, Due, Esplora, Yun, Tre, now Zero. Obviously we are getting a long way from the "The boards can be built by hand or purchased preassembled" claim that still greets you on the arduino.cc home page. Massimo said in an interview about the Zero that he imagined the user "would not even realise they were using a microcontroller", which gave me pause. Is that really something to aspire to?
When you work with a Diecemila or similar simple design, it strikes me that you can be in doubt that what you are dealing with is in fact microcontroller. And personally, I have no problem with that at all -- I think it's great, in fact! But my suspicion is that I may be on a complete different page there to Massimo and where Arduino may be heading.