[snom380]

In addition to the easy to install, easy to use IDE, I'd say the real innovation was a standard, idiot-proof way of programming the AVR with a well known bootloader.

From back when I was working on AVRs at university (2003-2004), I remember there were a ton of programming settings to choose from, different ways to program it (SPI, parallel), and you could easily screw up something if you set the wrong fuses, used the wrong oscillator frequency etc. All of this complexity was taken away by Wiring / Arduino, so you could focus on your project without digging through datasheets.

So the simplified IDE and social aspect was only part of the story.