Avalon bus - what makes it so special?

It is just a choice of words... when you design an SOPC system it is easier to see it as a bus, with all the masters and slaves connected to a same bus. But if you have a look at the RTL after compilation, SOPC builder in fact connects each slave to each master, meaning that it is fact an interconnecting architecture. You can even have two masters writing to two different slaves simultaneously, which would be impossible with a traditional bus.