All the reasons the guy listed for why the Arduino 'wins' are not unique to the devices. You can get all of those same things out of a radio shack basic stamp.
Arduino won because the stuck a decent microcontroller on a solid board (I'm ignoring the absolutely retarded pin spacing issue that pisses everyone off) at a decent price with a serial boot loader already burned to the chip. The ATmega chips were popular long before Arduino, so when it came out suddenly all of us who had been futzing around with ATmega's for years finally had a source for a preassembled prototype board rather than constantly cobbling our own together. I've still got several PCBs I etched with a generic prototyping layout in my shop.
They took the need for an Atmel ATmega programmer out of the equation but otherwise you get just a slightly larger than the chip itself prototyping board.
The Arduino software is complete ass, the only reason anyone uses it is because they don't know there something better ... like say ... entering your code from the command line with cat > filename && cc filename. The libraries, while relatively easy to use are painfully slow and bloated for no reason, which is important when your counting clock cycles on microcontroller.
Arduino didn't win because its Arduino, it won because it used a microcontroller that had already cornered the market.
There will multiple ATmega chips (the ones used in the Arduino) in every household before the Arduino came into existence.
Basic stamp from radio shack .. 100 bucks .. Needs a rs232 port for programming .. Who has a system with them anymore ?? Do you ?? I Dont .. I have to use a usb / 9 pin connector if I want old school serial. Include some stuff like a servo and some leds and resistors.
uno invetors kit from sparkfun .. 60 bucks .. any old usb cable Also includes "stuff"
That retarded pin spacing is a feature. Prevents you from plugging in a "shield" wrong ..
Also , and this is most likely the big reason they are doing so much better then the rest.
They are 100% .. completely open source. If you want a different pin spacing .. You can go and download the egale files ( or gerbers if thats your fancy ) and fab a board with your own pin spacing if you want .. Hell , you can even SELL that modified board if you want. If you really think its retarded , you can do make your own.