Re: Community and price.
Plus with that large community you get cool add-on boards like the PiFace Digital (which runs my garage door, with limit sensors and all) and a dual CANBUS board that talks to my electric motorcycle and runs a small dashboard and some datalogging.
I looked at Arduino, and it was US$85 dollars for a wi-fi shield. Ridiculous.
The USB power is a hidden plus, as it means you can run it anywhere you have cellphone charging, so I can use the cigarette lighter jack in my car or my motorcycles.