Parts quality is way better -- especially capacitors. But everything helps (resistors, hookup wire, volume controls, connectors, etc). And some of the new transistors are awesome, though the low volume of hi-fi leads to a lot of discontinued pieces of otherwise great silicon.
The further pursuit/emphasis on the "all out assault" side has driven the creation of some crazy (and interesting/awesome) high power units in each particular topology (e.g look at VTL's and VAC's top tube beasts, but also some of the crazy SET implementations) -- but that's basically an extension of "we have a huge budget so we can throw more hardware at the problem to get better performance". And very few can afford that. Computerized tube bias schemes too, I guess.
It seems that a few very bright folks have been pushing audio circuit designs forward (e.g. Nelson Pass, Kevin Gilmore), but elsewhere most products just seem like rehashes/remixes of ancient ideas (and not always done well, either). Or even original but really bad ideas.
That's why for budget-fi it's hard to beat the satisfaction of a good vintage tube amp rebuilt properly with quality modern parts (e.g. a pair of Heathkit W5 monos will turn out very very nice!).
